I totally agree with everyone who has said W3schools! It's a great place for new starters. Also I would recommend getting the developer tools plugin for Firefox. Using that you can see what CSS has been written for a site and also use it to show you what you're looking at on the screen as well as the written code.
Learning CSS isn't just about learning a language. It's about re-learning HTML itself. CSS will only get you so far without a proper document structure, and even then you have to avoid falling into the trap of designing the site using Internet Explorer ("But it worked fine in IE! Why isn't it working in FIreFox?") If you really want to learn about proper semantic HTML and CSS, you'll want to read Designing With Web Standards, 2nd Edition by Jeffrey Zeldman, before touching a single line of code. This book will help you "un-learn" everything you know about HTML and will get you started on the language's proper use and coding habits, which will make understanding CSS that much easier.