I don't care what anyone else says, most of what you will do with CSS you will learn through trial and error. I only know it because I practiced it daily until I got good enough to make a layout with it. If you want to test anything go here
It will let you dump code there to see if it turns into something. Good luck.
While a good resource, I can't stress how important it is to practice. It can give you basic concepts I hear (I didn't learn from w3, I learned from JigSaw before he left).
Also, don't get frustrated if you make a mistake either, you will likely make a few on your first few attempts, and even later on, you will try new things and make mistakes. The best thing to do is not to worry about it.