CSS Tricks is a really good site. It has a reference guide (not completed yet but getting there) and alot of video tutorials and articles to help you along the way.
the other good "teacher" you can use is view source.
every time you find a site you like and believe they've used css to achieve something.
click view source and see whats there.
and if you've got firebug (or similar) installed, it will break it down for you even further.
I do that some but sometimes it's a headache trying to figure out what they have done.