I've written one, and working on another. (I've also written a few novels).
Some books I'd recommend:
Lew Hunter's Screenwriting 434
The Screenwriter's Bible - David Trottier
The Elements of Screenwriting - Irwin R. Blacker
Making a Good Script Great - Linda Seger
There are two expensive screenwriting programs (Final Draft and Magic Movie Screenwriter), but as you're just starting out, I'd suggest the free route.
ScreenPro is a shareware Screenwriting Template for Word. (This is what I use and recommend)
http://www.passarella.com/screenpro/
There is also BBC's ScriptSmart
http://www.bbc.co.uk/writersroom/scriptsmart/
If you're looking for a place to connect with other screenwriters, check out:
http://www.triggerstreet.com/gyrobase/index
If you have any questions, feel free to ask.