Just write! No, seriously...just write. Sometimes we let our thoughts, what if's, and doubts keep us from moving on to the next sentence. Believe in yourself and that story that just won't leave you alone - allow it to come out in a stream of thought if needed just so you can get it on the paper. It's the rewrite and editing that shapes the book that everyone else will see. As a writer - you just have to get the ideas out.