My hard copy of the Children's Book Insider newsletter mentions the author, Jay Asher, and how he "entered his manuscript in as many contests" as he could. He's now written a very successful YA novel, Thirteen Reasons Why.
If you want to test the waters like I did a few years ago, entering writing contests is a good way to start. You learn about guidelines, word count, deadlines, theme, etc., while you're polishing your story for entry. Google or browse through some of your favorite magazines for writing contests. Writing contests are a wonderful opportunity for aspiring writers. Go ahead. Do it!