Write a lot. And I mean a ridiculous amount. You have to write so much that you don't mind throwing away and changing things that you've written - which is the second thing you have to do. A lot of young writers are very precious about their words. Don't be - you've got to be ready to burn stuff. You're not as good as you think you are, at least not yet. The more you write, the faster you'll write, and the less you'll mind throwing stuff out.