For anyone who wants to get started screenwriting but doesn't know how: http://mypdfscripts.tumblr.com/
Read. Just read. Horror, sci-fi, action, drama, films you love and films you hate. Read as many scripts as you can, and once you get a sense for what you like, get a copy of Celtex or Final Draft. Both programs will help you with most of your page formatting, but just in case, I'd bookmark this page, too.
The most important thing to remember is that STRUCTURE IS KING in screenwriting. The metaphor's a cliche, but it's absolutely true: a screenplay is a blueprint. It doesn't mean the finished product can't be a hut or a skyscraper or the Pentagon, but there are only so many ways to warp your materials before your building won't even stand up. This is a pretty good guide to the basics.
I'd outline your story before you start writing. That'll help you see where you're going before you get lost writing scenes. The only thing that goes in your action lines should be whatever an audience will see on screen. "Lionel quivers," is fine, "Lionel is afraid" is not. Show don't tell. Characters can be contradictory. People don't always say what they mean, and don't often talk about how they're feeling. Keep your descriptions as crisp and clear possible (again with the blueprints: they show every detail but aren't very colorful.) Flowery language dies on the page.
Write, write, write, and don't stop when it isn't fun anymore. The best work hurts.
Real nice, thanks for adding this.