I wish someone had given me a few tips about things I could do as a teenager to prepare for a career in writing. You don't need to learn to leap run-on sentences in a single bound, but doing these things will help your writing:
Practice. Allow yourself to experiment, learn, grow, and write some not-so-good stuff. Fill up pages and pages with your practice writing. Try all kinds of writing!
Keep a diary or journal about your own life. You may think your life is boring, but it’s not. Write about your observations of other people, your dreams, the sucky things that happen, the exciting things... Your diary will help you develop your unique writing voice. (And you might turn some things into stories later.)
3. Publish your writing. Work on the school paper or yearbook. Submit poems and stories to magazines. Enter contests. Create a blog. Review books.
4. Read, read, read. Reading will teach you how to craft your own stories—you’ll naturally learn what elements make good writing.
5. Learn. Take writing classes, attend teen writing workshops. Don’t worry if you’re the star student, or not (I've never been the shining storytelling student *sigh*). Your own passion for writing is what matters in the end!
6. Believe in yourself & follow your dream. Writing is a learned craft—and you can do what it takes to be published!
Teen Ink: http://www.teenink.com/Contests
Cicada Magazine: http://www.cicadamag.com/
Teen Book Blogger: http://www.booknerdblog.com
Teen Book Review site: http://www.teensreadtoo.com