Young people often ask me, how do I get a book published? My answer to that is most writers don't get started by publishing a whole book. They write articles, stories, poems and smaller things first and they try to get them published in magazines or online or they enter them into writing competitions. This is not only a good way to practice writing as you work your way up to something larger, it's also a good way of building up a resume to show a publisher when you eventually do have a book to show them. I was publishing smaller pieces for ten years before my first book got published. There are lots of great spaces both online and in print that are completely dedicated to publishing the work of young people. My favourite ones are Vibewire and Voiceworks.
I wrote a free ebook called 'So You Want to be a Writer?' which explains how you go about finding and submitting your work to places that will publish it. It has some great lists of writing competitions too!