The best way I learnt to write scripts is to read other scripts.
http://www.dailyscript.com is a great resource for that.
Another great site is
www.wordplayer.com which has a ton of articles about the craft. The guys who run it have written some excellent scripts including "Aladdin," "The Mask of Zorrow," "Shrek" and "Pirates of the Caribbean," so you'll definately get some good advice there.