The first thing I learned is that, when you're writing your first chapter, you will think your idea is the most awful thing you ever thought of, and that's okay. When I started my first chapter, I almost cried because I thought I was a horrible writer and I was never going to be an author. Then I started reading quotes by famous authors and I realized what I was thinking was normal and even classic writers had the same feeling. There will be many moments where you just want to give up because you think your writing or your story is useless but you can't give in, if everyone gave in, we wouldn't have "Harry Potter" or "Romeo and Juliet."
Second, just write everything that comes to mind for your first draft. Later, you will edit and refine your story and take out any meaningless pieces. Something you thought of could add a whole new aspect to your story you never realized before, so it's best to write it then in case you forget later.
Next, avoid using adverbs when you can; show, don't tell. Anton Chekhov said, "don't tell me the moon is shining, show me the glint of light on broken glass." I think that is the best advice I've seen so far that related to actual writing. That, and to not use semicolons- which I used in the first sentence of this paragraph, whoops.
Start. Marketing. Now. Don't give out your story unless you have an agent or copyrights, but start talking about writing or just put your name out there with other things you've written like short stories or a blog. Also, line up beta readers. Use them when you feel comfortable but make sure you have them reading your story before you publish it to look for plot holes and missing links.
Finally, don't get discouraged. If you're a young writer, like me, you do have the rest of your life to write this story, but why wait if you know you really want to write it? For teen writers, we understand better than anyone what it's like to be a teenager. We know what we feel and how we feel it. So what if you're twelve and only just started writing. Age and inexperience doesn't necessarily make you any less of a writer than Ralph Waldo Emerson or Veronica Roth. As long as you're confident in what you write, you'll do just fine.