Make sure your parents know and approve of your publishing plans. A literary agent will need your parents' approval before offering you a contract. A self-publishing company will want money to pay for the book. In either case, you'll have more authority with your parents' support.
Develop a name for yourself. Write articles for magazines oriented toward teens. Post comments on blogs or message boards related to the subject your book is about. The better known your name is, the more clout you'll have.
Identify the hook to your project. You're likely to have more success if your book draws on your own 13-year-old knowledge. For example, you'll have an easier time selling a book about teenage romance than middle-aged romance.
Research publishers receptive to teenaged writing. Teenage authors Jessica and Danielle Dunn published a book with Prufrock Press, which specializes in material for gifted children.
Write query letters to agents. A supportive agent can help you get a publishing contract, and it's not necessary to mention your age in the query letter. Consider going to conferences such as the Sewanee Young Writers' Conference at which you'll meet people who can help you with your career.