The best book I've read on scriptwriting was my first:
<a href="
http://www.amazon.com/exec/obidos/tg/detail/-/1582971587/qid=1041567533/sr=1-1/ref=sr_1_1/002-3417949-2516036?v=glance&s=books
" target="_blank">The Complete Book of Scriptwriting</a> by J. Michael Straczynski. He doesn't patronize you with "sure-fire" b.s. about the perfect formula. But he gives you screenplay formats (TV, film, radio, animation) and invaluable info on the process of writing and marketing.
Be careful with scripts from the internet. Not all of them are formatted correctly. Some have been mangled into HTML or text formats that aren't quite right. You can get the general idea of how they read, at least.
Some advice that may seem obvious, but what the heck...
Only write what the audience will see or hear. Putting in paragraphs about the character's backstory or inner thoughts is pointless. If the info is that important, find a way of working it into dialogue or actions.
Show, don't tell. Let a character's behavior tell us what he's all about, instead of using heaps of expository dialogue.
Don't number your scenes. That gets done by studio folks when the movie is in production.
Don't shell out for expensive software. You don't need it. Notecards, a couple notebooks, and a word processor should be enough. I use Word 2000 on my PC. I found the tabs to be a bit of a pain, so I bought the <a href="
http://www.screenstyle.com/screenxlar.html" target="_blank">Screenstyle</a> template for 30 bucks. Well worth it. You just install the template as an add-on to Word and it practically does the indenting for you. The instructions even give examples of screenplay structure to go from.