Once you have made your final decisions in what order everything is going to happen, you are going to want to take your stack of scene cards and start writing. Wait! Taking the time to do the following steps will go a long way to making your screenplay one of the GOOD ones.
Study dialog: Listen to the way people speak, and write it that way. Use contractions, use slang, do whatever the character would do. Just be careful in attempting accents; if the words are too mangled that they make the script difficult to read, you need to do some rewriting.
Read screenplays: There are many resources for free scripts online, and you can find them at your local library as well. Download some of your favorites, print them out and take the time to read them. Pay attention to how they are written, especially at any point you find yourself nodding along or getting excited about what you read.
Watch movies: Even bad movies can teach you things, such as what not to do. Watch as many movies as time and budget allow, and by all means watch them in the theater, where they were meant to be seen. If you can’t do that, at least rent them, but whatever you do, do not watch movies on regular television. By the time a movie has been "formatted to fit your screen" it loses much in the way of pacing and character development, and you loose a chance at a valuable lesson.
The cardinal rule of writing applies as much to screenwriting as any other kind; if you want to be good at writing, the write. Study, read, watch, learn as much as you can, yes. But make sure that you spend more time writing than any of these things, and soon you will have written one of the good ones.
Some great books to further learn the craft of Screenwriting are:
Screenplay: The Foundations of Screenwriting, By Sid Field
Screenwriting Tricks of the Trade, By William Froug
Zen and the art of Screenwriting, By William Froug
The Complete Book of Scriptwriting, By J. Michael Straczynski