Steve Krug is a major name in usability and usability testing. He’s written a number of books on the subject. If you can get a hold of it, you should really check out his book Don’t Make Me Think.
Here are a couple of lessons from the book that will be useful to you when thinking about site layout and design.
Also, check out this site. It’s a good resource on usability and user’s interactions with sites: http://websitetips.com/usability/