If you are in your teens and want to write then more power to you! This is the best time in your life to begin. The more you write now the better you will become. There is a huge market for books written for teenagers so why not have those books written by teen writers? They are in touch with what the kids their age are feeling, experiencing and dealing with and you can't get much better than that.
If you want to become a teen writer than don't let anything stop you. Do your homework and then get writing. The best place to begin your research is with books written for teens. Read these books to find out if this is the kind of writing you would like to do. You don't have to write to just the teen market but it is a good place to start. Read other books to see if you feel more akin to mysteries, science fiction or even romance. The more you read the better writer you will become.
You might come up against some opposition because you are a teen. Don't let your age hold you back. When you submit a book to a publisher you don't usually list your age or birth date although you will need to submit that information once you are accepted but by then who cares! Use the resources that are at your disposal. Teachers, counselors, classes and courses are just a few. Find a mentor and have them guide you through the process. You can find your own or ask one of your teachers to help you to find one.
Start out with writing for your school newspaper or other scholastic publication. Write letters to the editor or articles and short stories. You can submit them to student publications, magazines aimed at teens, newspapers in your community and so on. Get involved in writing contests in your area and even writer's groups. If there aren't any in your area start your own.
Buy or check out books from the library that are made just for those of us who want to write. Everything from style books and dictionaries to Writer's Market and research publications. Read voraciously and pick up new words for your vocabulary as you go.
Do everything your adult counterparts would do to learn how to write, get writing and then get published.