What do you do if you are a kid who wants to write? My advice would be do it. If you feel like writing, then write. Write letters, write poetry, write in a journal, write stories. Any kind of writing is good practice if you want to be a writer. And read, a lot. Read the kind of books that you want to write. You need to become familiar with the flavor of what you want to write.
Of course it doesn't hurt to read other genre's too. Get lots of information crammed inside that head of yours. You may love science fiction, but give a historical novel a chance.It doesn't hurt to have lots of insight into other things to glean from. Historical facts, biology, science, language, geography, astronomy, mythology, all these subjects and more can be helpful in writing. Learning is great adventure. Barbara Streisand in her movie "Yentl" sang,"The more I learn, the more I realize -the less I know." I believe what she meant is, every time you learn something, you realize that there is so much more to learn. Then take what you learn, use your imagination, put a new twist on it and tell a story that no one has ever heard before. Albert Einstein, one of the greatest thinkers of our time, said, “Imagination is more important than knowledge. For knowledge is limited to all we now know and understand, while imagination embraces the entire world, and all there ever will be to know and understand.”
So get out there and write. I dare you.