I started with http://w3schools.com. Make sure you're using Firefox and the Firebug addon. Get your hands dirty then get familiar with the web design community.
I have CSS Mastery by Andy Budd on my desk and it's a good, readable, short, yet deep guide to CSS.
Don't Make me think has also become my mantra of web design.
Overall, you're going to produce a lot of crap--as I have--before you get good. If you have someone to look over what you're doing that'll be the best help. Personal drive will matter the most in the long run though, so stick with it and keep learning.
I have to agree that it's a lot of trial and error. There are sometimes many ways to achieve what you need to...some more "proper" than others. CSS Zen Garden was one of the first sites I discovered that really helped things click in my own head. View the design, view the source, view the design, etc...
A couple of books I appreciate having around are Dan Cederholm's Bullet Proof Web Design and
Andy Budd's CSS Mastery
Both give solid ways of accomplishing some common techniques and issues you'll run into with css-based designs.
Css Mastery - is the best CSS book I have read. If you think you know a lot about CSS, this book will open eyes!