Establish your credentials as an author by building up writing credits. Practice craft and submit to magazines that specialize in teen writing. "Merlyn's Pen" showcases fiction by writers in 6th to 12th grade. "New Moon Girls" magazine is edited and written by girls age 8 and up.
Participating in contests and online forums can build up followers, also known as a platform. Teen Ink has a forum on which teenage writers post stories and other writers critique them.
Widen your definition of book publishing. The major commercial houses are competitive, but smaller, specialized presses may be more receptive. Anthologies, chapbooks, and self-publishing are other avenues to publishing a book.
Agents work with authors to help them sell their books. Some literary agents represent teenagers. Look through directories and web sites to find agents who are looking for work. Also look through acknowledgment pages of books similar to your own.
Successful books by teenagers tend to draw on subjects where teenagers can bring their own unique insights. S.E. Hinton decided to write a book about gangs after she saw a friend get beaten.
Christopher Paolini's parents self-published his book in 2001. Best-selling author Carl Hiassen happened to read it, and showed it to his publisher, Alfred A. Knopf, which went on to publish "Eragon."