Writing is writing, but I am not confident enough to take on creating a whole world from scratch. Will writing fanfic or writing in other shared world spaces be beneficial, or will it simply allow me to get into some bad habits?
As long as you approach it with good writing practices and treat it as you would your own real writing project, it can help you practice the art of putting words on paper.
What it will not prepare you for, however, is world-building, which is the other half of the battle when you write, and is just as important as your ability to write. You can be a brilliant wordsmith, but without a story to tell and a world to tell it in, your talent is wasted. At the same time, you can have the most compelling story ever conceived, but if you don't know how to put it in words, there's not much you can do with it.
In short, writing fan fiction only partially prepares you for the "real deal".
In general, I feel that fanfic is a crutch that keeps a lot of people from honing their craft and moving on to original fiction. After years of NaNoWriMo and some side work with an editor, I've only seen one person start with fanfic and graduate to doing their own, thoroughly original work.
It's like they're just too scared to ever leave their comfort zone, and why wouldn't they be? The fanfic community tends to do its own thing, not overlapping much with other types of writing, so it's easy to feel like a big fish in a small pond -- to go from there to being a guppy in the ocean of original fiction is probably a jarring experience to say the least. Plus, if you are writing fanfic, we all know you aren't writing for publication, so it's not like not getting published would be a failure you might have to cope with. By not trying, you feel like you can't fail.
Now, I said "in general" at the beginning, and I meant it. Here's the exception:
Once something has found its way into the popular consciousness, and simmered there awhile, it can be the basis for a new work that, if not original fiction per se, is at least legitimate and useful fanfic. Some examples that come to mind are the Disney retelling of classic fairy tales, Wicked (a retake on the Oz world), and Treasure Planet (not sure if this is Disney or not, but it's a resetting of Treasure Island in the future, in space, with a cyborg).
The difference here is that instead of creating fanfic of Harry Potter, Twilight, or some other thing fresh off the presses (which in addition to the issues above does not make you look like you are serious about your craft) -- the authors took something that has had a generation or two (or more) to sink in, and instead of trying to ride the coattails of other storytellers, the authors have used the archetypal status of the characters as a tool for their own craft, playing on our preconceptions. Or, the work has been made more accessible to a new audience:
Adults who have read the original Alice in Wonderland see the Disney cartoon in a different light than children seeing the cartoon for the first time.
I'm not sure Wicked even makes as much sense if one is not at least conversant in the original Wizard of Oz.
Treasure Planet was mostly an update, both for new cultural norms in how we tell stories to children (not that my son doesn't get to hear the originals, but I know we aren't the norm) and because I'm not sure a kid born in the age of GPS units, Coast Guard cutters, satellite beacons, etc. is likely to find the same danger factor in Treasure Island that someone born in the book's time would have.
"Well, thanks for stranding me on this deserted island, but I have a SPOT with me, so I'll just turn on the emergency beacon and tweet to all my friends about what a traitor you are. The Coast Guard will be here to get us before I run out of Snickers bars." -- Yes, I can tell my little one that there were no Coast Guard, satellites, GPS, or Snickers back then, but he won't really grok it until he's older.
The problem with fan fiction is that it will always be tied down to the source material, and can't really become more than what it is. They're a sidetrack rather than a stepping stone.
Okay, to reword that first paragraph:
If you are going to draw from a source material, be aware that you are creating a branch of the source material. You're creating something that is anchored in someone else's fiction. As HedgeMage illustrated in her answer, sometimes this is a good thing in specific cases, but in general "fanfic" use it leads to bad habits.
If you find yourself having trouble creating your own fiction, ask yourself what it is specifically about this source material that you like. It won't help to wholesale copy characters/settings/etc from a source, but it's certainly okay to find inspiration from the elements that compose your favourites.
If you're having trouble world building, perhaps you can draw inspiration from real world locations. Every fictional city/street/country draws something from one or more counterparts in the real world.
I know many people who started out writing fanfiction and then turned this into the basis of a professional career. Off the top of my head, both Paul Cornell and Una McCormack both started by writing fanfic and are now professional authors.
Unfortunately it can be all too easy to spend all of your time there. If you think you'll enjoy it, write fanfic by all means. You'll certainly learn lessons from it and be able to take that knowledge forward into your later, original work.
I'd urge you, though, to remember what your aim is, and that's to write your own worlds.
Fanfic can be a great training ground for a writer; but it can also be a seductive echo chamber. Good fanfic, like good everything, is rare, and will receive a lot of praise. And leaving the nest can be a harsh experience if you're used to praise in the fanfic world, and find that you need other tools in your writer's toolbox to make it on the outside. Many people never write anything other than fanfic, and that's fine, but from your original question it seems you're aiming for something more than that.
At some point you'll need to start writing original stories anyway. Whether you decide to write fanfic to get some experience first or not, I'd urge you to start on your own work sooner rather than later.
It is not a crutch. Writing is writing.
As Jeff Atwood put it:
The process of writing is indeed a journey of discovery, one that will last the rest of your life. It doesn't ultimately matter whether you're writing a novel, a printer review, a Stack Overflow answer, fan fiction, a blog entry, a comment, a technical whitepaper, some emo LiveJournal entry, or even meta-talk about writing itself. Just get out there and write!
If fanfiction is what gets you writing, then write. It doesn't mean you have to do it for the rest of your life! But whatever you are interested in doing enough to get you writing is good.
As a parallel, my mother wanted to discourage me from reading comics when I was a kid, but my father argued that anything which got me reading--even if it was reading with pictures--was a good thing.
People here have variously argued that fanfiction is a crutch, that it is limiting. But that is silly. It's limiting in the sense that you couldn't go and sell it, but other than that, it's an art form like any other. Some people here claimed you couldn't do world-building with it; that is wrong, because you can always build on top of the existing world the fandom is set in. You can always add characters to the ones already in the fandom, and you can always take the characters in a direction that was not obvious from the show, so good character building is also possible.
In short, there is nothing you can do in any other form of fiction writing that you can't do in fanfiction, and any skills you hone while writing the latter will help you in the former.
I have encountered numbers of excellent fanfics out there which could stand out on their own, and be excellent works by themselves if the author just didn't constrain himself into the pre-created world.
I'm not saying that you should always create your own worlds, but when you're confident about your work and people start to appreciate it, it might be a good time to whip up your own universe and make a recognition for yourself.
It's a crutch.
The scariest thing about writing is the idea that nobody will like (or even understand) the stories and characters you create. Writing fanfic is an attempt to dodge that risk by using stories and characters that are already well liked and well understood instead.
The problem is that you never learn anything without taking a risk, and so by hiding behind other peoples' characters fanfic writers stunt their own development. The way you learn how to write good characters and compelling stories is by writing bad and boring ones. The things you do wrong give you insight on how to do it right. But this never happens in fanfic, because you always have that world's established tropes to fall back on.
In other words, to tell great stories you have to be willing to fail completely, and fanfic writers aren't.
In theory I have no problem with fan fiction, however the fact that the fan fiction community's standards are absymally low is enough to dissuade me from ever associating my work with them. Using another writer's characters, setting, and themes is perfectly reasonable though and has been used by professional and accomplished writers throughout history. I would just avoid categorizing anything as "fan fiction."
I think many write fan-fiction simply because they're inspired by a universe someone else created. I have no doubt it can be a crutch, but it doesn't have to be.
Perhaps you should ask why you're writing. If it's simply because you enjoy writing or because you want to explore a universe, then fan-fiction can be a great outlet.
Someone asked in reddit whether fan fiction can be "taken seriously". Here's what I said - much of the same points as others are saying:
Go ahead and try. Simplest advice I can give is just that.
But if you really want to get ahead with writing, go ahead and try some more stuff.
The only big problem with fan fiction that I've seen is that you're always in someone's shadow. Here's the Great Author, there's the Fan. And the Fan has to make a choice: Are they going to remain a Fan forever, or try becoming an equal to the Great Author. And the Great Authors are just fine with someone else becoming as famous and awesome as they are, but they're not cool with them doing that with their intellectual properties. Hence, fan works are likely doomed to obscurity. Fans tend to stay in their place, whether that's appropriate for them or not. In a more sane world, open universes would be the norm and fans would always be equal to other people writing stories based on those universes, but we're not living in such a world.
This isn't to say that writing for someone else's universe would be too bad in principle. It's just like writing things based on real world and history: it may need interesting research work to make your story fit with the rest of the established world.
And as far as trying stuff goes, designing worlds from scratch isn't very difficult - and can be quite rewarding.
And as far as not having confidence trying to design worlds from the scratch goes: The dirty secret of world design is that a) it's really not that difficult and b) it's almost criminally fun when you get started. And like characters, the milieu will just keep growing when you tell the tale - it's useless to design everything in advance.
I think fanfic can a very good training ground. They have provided characters, settings and most importantly audience. That lets you focus on hone your other skills.
The biggest advantage is that it has a built in audience that tends to review what you've done. You get feedback and that is the most valuable thing you can get.
Think of it as a form of deliberate practice. You can practice dialog with existing characters or describing existing settings and so on. You can even create new characters and settings and insert them into the story.
I think fanfiction is a great start if you enjoy writing. There's nothing you can gain from writing completely original stories that you can't gain from writing fanfic. As for no confidence in creating your own world, with fic, you can build on that confidence. You could try putting some of the characters in a world not mentioned in canon, an AU, which would improve world building. Also, you could create original characters in the story. Once confidant doing that, you could tell the fanfic from the OC's point of view about the main characters. These are things that won't hurt a writer to learn.
When you are first attempting to write fiction, fan fiction is a good way to practice. It gives you existing characters and a world setting so you can focus on drama (or comedy), characterizations, writing sentences that make sense, and creating a story others can understand.
Please note, your first attempts will suck and will most likely (and deservedly) end up in the trash can. Expect to spend a few thousand hours writing crap before you produce anything worth showing to others.
After you have cut your teeth writing some fan fiction, you should ask yourself a question and be honest: Are you writing for the fun of it, or because you want to create something publishable (or at least something others will take seriously).
If you want your writing to be taken seriously, then quickly leave fan fiction behind. Good writing requires mastering a wide variety of skills, such as creating interesting characters from scratch, building your own world with its own mythology, and so forth. Fan fiction won’t help you in those areas, except perhaps as initial inspiration. Relying too much on fan fiction will retard those skills and make it harder for you to branch out into other areas.
It's a crutch if you don't make it solid enough to stand on it's own. Write good stories that can be read in any context and you'll be able to mitigate that.
Don't let yourself get stuck in niche fanfic cultures. Don't write stuff that requires people having read every other fanfic on the forum to understand. And don't let fanfic be the only type of story you ever write.
Understand that you're just choosing the constraints and backstories of your world. People that write non-fantasy use the real world as their source material. It's not a crutch for them, it's a choice.
I am a bit of an amateur writer and I have always considered fan fic to be the bottom of the barrel. The people who write it are generally those who have obsessed over some one else's work. They may re-arrange the concepts and plot, but they are creating nothing really new. Most are also so inept at writing that in truth they detract from it.
As some one else mentioned, world building is a large portion of writing fiction. Without those skills, you are ruining your own potential as an author. Honestly you should take the time to write completely original short stories. Share these stories with a few friends to get their honest impressions, then toss them aside and work on another. After a few months, once the story is old and mostly forgotten, re-read it yourself, and re-write it.
I don't know if Fanfic is a crutch or useful practice, but it is fun. For me, writing fanfic is one of the ways I enjoy a book. So I'd do it whether it's useful or not.
That said, can writing fanfic be useful?
- Fanfic allows you to engage in "what if" scenarios: what if Frodo and Sam were lovers, what if Draco Malfoy was secretly good, whatever. (I'm deliberately choosing infamous examples here.) "What if" scenarios are useful for when you write your own stuff too: when you're stuck, you can ask yourself "what if this happens", when something doesn't work you can go "what if this happens instead of that." Such flexibility is necessary to arrive at a good story.
- You (hopefully) learn to realise the limitations and the conventions of the characters and the world: when a character is acting "out of character", when whatever doesn't exist in this world, when Captain Kirk can't die because the world will change its basic laws just to keep him alive. In turn, you can then learn to realise the limitations of what you write, and consciously use them, or avoid making them.
- You can learn to recognise when you're writing something that's objectively bad, but for one reason or another really fun for you. For example, you might have inserted yourself as a Mary Sue who goes on to save and marry the main hero of the franchise. Having learnt to recognise this bad practice, you can keep it to your drawer stash and away from material you try to publish.
- Utilising all the above, a fanfic story can be good. You can use the established and widely familiar world to draw attention to something, in which case the fanfic aspect becomes a strength of the story rather than a weakness. For example, in Harry Potter and the Methods of Rationality, Eliezer Yudkowsky deconstructs the whole Harry Potter series by stressing multiple logical failures in the original and suggesting how smarter characters should have acted. He is also quite funny in the process. This is achieved by asking "what if Harry Potter's aunt and uncle were nice Oxford professors instead of schmucks" (point 1) and by noting the limits of Rowling's world (point 2).
Doesn't it all depend on whether you intend to show anyone else this fanfic? If you write a story using someone else's characters or world just for your own private practice, surely that is useful for you and does no harm?
It might help to avoid worries over creating worlds, characters, or even a plot, if you just use an established starting point you are familiar with, and write from there as practice. True, you won't get any practice at creating characters, but that is just one of the many facets you need to work on.