Don't be afraid to write crap, and don't be afraid to skip around. The first draft is all about chasing your interest. If there is a scene in the book that you're looking forward to writing, but it is taking too long to get there, just jump ahead. The last thing you want to do is kill momentum, so skip, hop, and misspell your way to the finish line. You can't edit something if it isn't there. And, as Stephen King says in his book 'On Writing', which I highly recommend, make sure you write the first draft with the door closed. Don't let anyone else in on your ideas and drafts. Outside input is just going to make you second guess yourself. Write the story only for yourself first, and only during the revision process should you let others in.