So, I have an idea for a book, however I have been having trouble getting this idea down on paper. I can visualized the various scenes in the book, and have written up and drawn a diagram of the plot, however when I get to actually writing down the story I come to standstill. I begin to fret over which words would be the best to use, and worry that each senetence is not as good as it could be.
Every accomplished writer sees tons of changes and edits applied to their manuscript before it's published. And these are people who've been in the business for years. As somebody who's just starting out, don't fret it. Because that's a surefire way to slow your progress and fill your head full of doubt. My advice to you is to just get something on paper. It won't be perfect. It never is. Every author in the business writes multiple drafts of every novel.
Just keep those fingers moving across the keys, keep that pen moving across the paper, and finish the story that's in your head. You might find yourself wincing at some of the stuff you write, but that's okay. This is just the first draft. Once you finish your project, you can put it away for awhile and when you're ready you can come back to revise it with a fresh state of mind. So don't fret over those individual words and sentences, especially on the first draft. You can always come back and fix them. Just finish your draft, pat yourself on the back then get back to work.
I'd like to recommend a book to you that I read recently:
Techniques of the Selling Writer by Dwight V. Swain. It's an older work, and shows its age in some places, but the advice is really good. You can get a copy on amazon.com for a few bucks and I think it'll really help you out.
Best of luck!