Oh boy, this one will have a thousand plausible answers. But I'll do my best to help as I can. There are, of course, a plethora of options, so please do not think this answer is the be-all end-all of it.
So let's try breaking this down.
If you are writing fanfiction (based on the works of others), you can publish on http://www.fanfiction.net where there are likely millions of published works. Note, though they are 'published', they are not meant for monetary gain. In fact, it's apt to be illegal (making money from the Intellectual Property of others), or at the very least unethical. But, there are many examples of novels that started as fanfiction of other works (Twilight, Fifty Shades of Gray, what have you).
There is also https://archiveofourown.org/ which started as a response to fanfiction.net's crackdown on... shall we say certain content. They do have to 'approve you' as a writer on their platform, but as far as I know it's merely a waiting game. I'm unaware of the typical content or the quality thereof on the site.
If you are writing original fiction, http://www.fictionpress.com (affiliated with fanfiction.net) can be a wonderful platform for it. I cannot tell you how it compares to fanfiction.net, because I am not as active there.
There are, of course, scores of other options, including hosting your own website. Just google "free website hosting", and sure you'll find lots of information. If all you mean to post is stories, I doubt you'll have any issues with even 20MB is data, but that does mean you will have to get the word out yourself. I've found and read lots of excellent stories hosted on independent websites, some of which remain my favourites of their genre.
Next comes the NaNoWriMo contests.Check https://nanowrimo.org/ for one (not the only, but certainly a good place to start). It stands for National November Writing Month. It's a challenge for all writers to complete a novel (typically in the range of 50K to 70K words) in the month of November. There is a March 'drive' as well, but that's not as common. The point of this community (/challenge) is to get a lot (and I mean a LOT) of writers working on their project/novel together, so we can keep each other motivated.
Some competitions offer cash prizes, but not all. The point is to get writers into the habit of thinking in terms of novels (as opposed to stories), and help each other stay motivated.
Now, onto more paid content. While this shouldn't be your first step, it is a logical step and one many try to take. What you can do is look into writing competitions. They usually have a set genre/theme and often your target audience is laid out in the prompt as well. There are most often cash prizes, but it is a competition, so you are pitted against others.
There are also writing commissions you can look into. Think magazines, online publications, and the like. Google will take you far, just know that it isn't easy to turn a hobby into a workable income.
If you want to self-publish, you can look into publishing for Kindle, Amazon, and other e-book centres like that. On their websites you can often find all the data you need for it, and there are plenty of authors that have self-published willing to explain the ropes.
One example I've found, but haven't delved into (because I don't want to take that route), is Jenna Moreci. https://www.youtube.com/watch?v=OUOBQwlQ480 Is a video of hers talking about it. She's self-published two books I am aware of (The Savior's Champion and Eve: The Awakening), and is (was, seeing as she's currently on hiatus) a regular on YouTube. She also talks about a lot of things about the craft of writing, if you are interested.
I, however, am far more interested in the traditional publishing route. That means finding a literary agent, who will approach publishers and deal with all the crap I have no patience for. I could give websites and the like, but simply googling "literary agents" and adding your genre (fiction, nonfiction, fantasy, etc) will point you in the right direction.
In this I can offer you far less guidance, because of my niche (Young Adult and New Adult and Adult Romance fantasy with LGBT+ characters, which is a bit trickier than one might think). But you should find what you're looking for easily enough.
Be aware that it's difficult to 'get' an agent, because you need to query them. They need to be available to represent you, be interested in the genre and target audience, and be interested in your current project/novel. It gets more complicated still, because not all of them will tell you why they reject your project--this makes it frustrating when trying to figure out where you are going wrong.
Full disclosure. Though I am not actively querying, I have had my fair share of rejections. It's disheartening, but I am not giving up that easy. I take every 'no' as 'you need to try harder', and have used that as motivation to continuously step up my writing game. (Personally, I can see no downside to getting better, and there is nothing that will stop me from continuing my journey to becoming a better writer).