Can't recommend Mark Rippetoe's Starting Strength enough here -- he's got an entire lengthy chapter devoted to squatting -- you'll learn a ton.
I've read lots of people on here (including me) talk about how much they realized they were doing wrong after reading Rippetoe, and how much better they got at squatting afterward.
In the meantime, you may want to check this out*:
http://startingstrength.wikia.com/wiki/Squat_Videos
* Props to LisaS!