If you have no experience writing or story-telling, my first advice to you is this: Instead of worrying about how to be a perfect writer when you do start writing, get some experience instead. You can know all the rules you want, but that's not what I believe usually helps a person who's just starting out (a lot of the rules are just convention or people's opinions anyway). You need flow. You can always edit or write another draft later. Enjoy yourself.
Of course, studying writing rules is great, and it's fine if you want to do it (even at this stage), but it's a lot like taking a Spanish class where you just learn about grammar and the best way to talk (according to a set of rules). You're not going to become fluent quickly that way (although knowing how to conjugate verbs may really help you out some day). You actually have to practice talking—and practice a lot.
My second piece of advice to you is to practice writing what you're going to be writing. Short stories and novels are different animals. True, writing short stories may help some with the writing process in general, but you'll still have to learn to write novels some day—and it's not the same thing (although exactly how different it is depends on how you write). Feel free to ignore this piece of advice. Just writing something is better than nothing. If you're good with short stories, do that. A lot of novels have several sub-stories, anyway. If you can fit them together somehow, that might work.
My third piece of advice is something that really helped me when I was younger. Here's the back-story first: I wanted to write novels, but I was having a really hard time doing it how I wanted. Things weren't coming out right, notwithstanding I had a plan for how things should be. I had written a long story a year or two before this point and had great success in actually doing that writing (writing it enjoyably, yes—writing it in perfect English, and following a preconceived plan, no)—but, this time was different. There were some problems that I see. I was being a perfectionist about grammar and getting things right (I didn't do that at all the first time.) I was too careful. I didn't realize that most novelists write a number of drafts (and have editors do a lot of work on the final one) before they come out with a great novel. They also typically do a lot of submissions to publishers before their work gets accepted to be edited (if you go with a traditional publisher). Another problem I faced is that not only did I lack experience in writing, but I also wasn't very well-read (let alone in the genre of my choice, although reading in the genre you write in isn't necessarily the best idea—some of my favorite writers back in the day weren't into that).
Anyway, so that's the back-story. What helped me, though (not with my ideas, per se—but with my actual writing), is quite interesting. Sadly, I stopped writing for a few years—but I did something great during that time that helped, a lot! I read lots of books (books that I liked). Then when I started writing again, I could write fiction (and even poetry, which quite amazed me). It was magical. The writing flowed from me. I think being lonely and depressed helped, too (but I don't recommend trying to become lonely or depressed). After I got some experience, that's when I got more interested in writer's groups where they tell you rules like, 'show—don't tell'. (I found a good group in the form of an emailing list, and I learned a lot from them.) You don't need to worry about that when you're first starting (when you have no experience), at least if you actually want to be writing stuff, too (but do develop good habits—or else be willing to break the bad ones when you learn they're bad; that might get harder as you get older—so don't get set in your ways; that's probably something I need to think about more).
My fourth piece of advice (another thing that helped me) is this: Draw a map (especially if you write fantasy). It can help the writing process (and ideas, too, as you've got to explain the stuff on your map). A map can help to connect things together, and I think it can help the flow of your writing.
Planning individual characters, and what their personalities are like, helps, too, in my opinion (if you haven't done that, yet).
My next piece of advice is to do one of the following:
- Write on actual paper with a pen. You won't be able to edit or overly focus on perfection if you do this, and you won't be distracted by the other stuff you can do on a computer.
- If you write on the computer, make sure there are no icons on your digital desktop. If you're like I was some years ago, they can really distract you subconsciously, and get you thinking about things you need or want to be doing that aren't writing. Clearing them away (or disabling the desktop altogether) can help one to focus.
There's also the digital version of writing with a pen: commit not to delete anything (or else use an editor where you can disable deletions). If you can't delete it, you may also subconsciously take what you write more seriously, and do a better job as a result (because you'll get used to being better at doing it right the first time, without being overly thorough, second-guessing yourself, etc.) Second-guessing yourself isn't a very assertive thing to do (and it's good to know how to be assertive, effectively, as a writer; in my opinion, it can help you to convince others, and make a bigger impression on people, including, in my opinion, publishers, even if you have a few glaring flaws)—but don't be assertive about things you know nothing about (even if you might convince people). Making a strong impression based on inaccurate information that you're assertive about won't win everyone over (even if it might get you a following—and some enemies to boot).
In summary, my advice to you is to practice writing. Read a lot (even if you're not writing—it adds up). Study grammar, writing rules, etc. (but don't take them too seriously, or they might just get in the way). Realize you probably have a lot more drafts to write anyway. Do things like drawing maps to help connect the dots. Write novels as you practice.
Now, this is my advice. Someone else can give different advice (I haven't actually read the other answers—that's a bad practice, I know, but I wanted to write mine). Try what works for you—and, don't let rules (including anything I say) hold you back. Don't let a critique dampen your style. Listen, yes—but don't stop writing, even if you have to ignore the critique. You'll learn how to be a better writer in time as long as you don't give up, and as long as you keep learning. Pay attention to the writing styles in the books you read, and also to what rules they follow—it helps. Traditionally published writers aren't perfect, either (but they combined with their editors do a really good job—I tend to think the editors are the ones who really make a book shine with that look of perfection most of the time, however: all kinds of people write books, and they're not all 100% perfect; read some pre-edited manuscripts, or some self-published books on Amazon—you might just see what I mean). Editors are important for a published work, but you don't have to be the final editor. You can be, but you really, really need to know your stuff. Lots of people think they can be editors (but, I don't think they're all ready for it, just yet). Again, even unedited, self-published books with all kinds of issues can become popular.
Writing something is important.
I was going to end there, but I just have to tell you that when you actually do write stuff, don't stop there. Don't focus too much on what you've written. Keep writing!