So you have a great idea for a movie. It's going to be great! What most people will tell you is to just start writing it.
Well, I don't entirely agree with that.
True, you have to get going as soon as possible, and writing as much as you can. However, just sitting down with Microsoft Word open is setting yourself up for some challenges.
First thing you need to do is write yourself a basic idea of what the story is. You may not have it all figured out, and that's alright, but write down what you have. Maybe doing this will help you figure out the rest of it. That's what happened for me. Don't worry too much about dialogue or big descriptions unless it's really important to the plot. This is going to be your outline.
You'll often see structures for outlines, telling you what to put on which page and things like that. Don't worry about that. Those outlines are for people that are pitching ideas. You already know you're writing it, so this outline is for you. Use it to gather all of your ideas together. It can be as short or as long as you want, but I'd recommend keeping it around 3-5 pages. It's meant to be organized, so if it becomes a sprawling short story the point is slightly defeated.
Once you have that, and you know where you're going, don't start writing until you can do it in proper format. Eventually you'll want to switch to proper format, so you may as well start there and save yourself the trouble. Most writers use programs like Final Draft, but they are expensive. There are free screenwriting programs you can get that are surprisingly full featured. The most popular is one called Celtx. Take a little time to figure how to use your preferred method, then get writing!
I'll post again to talk a little about story structure and the creative side of things, but this should get you going. Again, this is what I did when I wrote MY Script. There's plenty of other ways to do it, and no way is right or wrong.
Below are some useful links, the first is to the free program CELTX, then two paid but great programs, Movie Magic Screenwriter and Final Draft. Clicking links from this site helps us make the movie, so please feel free to check them out!
Movie Magic Screenwriter Version 6
Final Draft Version 8