Read them from the top down (in order)

Save The Cat! The Last Book on Screenwriting You'll Ever Need

Screenplay: The Foundations of Screenwriting

How Not to Write a Screenplay: 101 Common Mistakes Most Screenwriters Make

Making a Good Script Great

Writing Movies for Fun and Profit

Screenplay Links

Wordplayer.com 

JohnAugust.com

Things to avoid in your screenplay

Without a Box