As I write this, keep in mind that everyone has their own writing style. Some like to write out a detailed plot summary before they ever begin the first draft. I'll blog about that later; I'm not really in the mood right now. Others like to sit down and just have at it, fixing all the plot details later on their second or third pass. Again, I'll talk about that later. What I'd like to talk about right now is the first draft.
The very most important thing you should keep in mind about your first draft is that it's supposed to stink. No one can create the perfect first draft. It doesn't exist. The first draft is the place for you to throw all your thoughts up on the screen and get them out in the air. You can come back later and reorganize them. Right now is your time to spew. It doesn't matter if your spelling is awful, your grammar horrid, or if you use "but" fourteen times in the same paragraph. Fix all that later. Right now just get the story up on the screen.
The American Night Writers Association, or ANWA, calls this "writing dreck." It doesn't matter if what you've written is even any good. Just getting it down is the important thing. You can go back and fix it if it's written down. But if you've written nothing, there's nothing there to fix.
So I challenge you. Sit down and for a half an hour, just write like crazy. Anything that comes to mind. Can't think of a character's name? Call her Sally for now and get on with it. Then come back and take a look at it. I think you'll be surprised to find that what you've written isn't that terrible after all.