I want to publish a book, but can a 13-year-old really do that?
Yes, you can publish a book at that age, as it is possible. This is due to the fact that there are no specific age qualifications for one to become a creative writer. As long as you have someone like a family friend or older person to help you through the process, it is possible. You can get tips on how to go about it at, http://www.wikihow.com/Publish-a-Book-at-a-Young-Age,or at http://www.ehow.com/how_107987_self-publish-book.html.
Amazon has a self publishing service that you might find interesting.