Google NaNoWriMo (in the interests of full disclosure, I participate in what they do but I don't own that site and I don't make any money suggesting it) and look up their Young Writers Program.
NaNo is good because it adds a social aspect to what can be a very singular experience. Plus you get people who actually know what they're talking about.
Don't want or like NaNo? Then try writing groups on Facebook. Quality will vary considerably. Stay away from places where the only purpose is to advertise.
Don't like Facebook, either? Then try the #amwriting hashtag on Twitter. There are a number of participation games, such as #Chance2Connect, #SciFanSaturday, and #AuthorConfession. Start to answer and participate in the community. People tend to be really nice and they will encourage you.
Another suggestion is to seriously consider a writing course. Yes, I know, lots of people just start writing. And there is nothing wrong with that! But you seem hesitant and that's why I'm suggesting it. Plus, a lot of people who plunge in with no preparation end up with a mess of editing at the end.
Don't want to take a class? Then read either Ray Bradbury's Zen and the Art of Writing
or Stephen King's On Writing.
Read a lot, both in your genre and out of it. Write a lot of side stuff, such as short stories, haiku, a quick character sketch.
And, by the way, the first million or so words that you write will stink. That's nothing personal. It's just the way things are. You need to get those million words down on pixels or paper because that gives you experience.
You can also try Wattpad (again, I don't own Wattpad but I do participate there and I am also an engagement ambassador there) but be prepared for not necessarily getting a lot of people reading your stuff. The demographics there skew heavily female, 13 - 28, and American, Canadian, or Filipino. So they don't necessarily dovetail with the kind of story you might want to be writing. But some things take off. It can't hurt, and it'll give you some experience taking criticism, a thing that all writers have to deal with.
And, finally, my writing teacher told me. Characters --> Conflict --> Crisis --> Change. Stories need all four. Good luck!