There are plenty of CSS tutorial sites on the net; some are aimed at beginners, while some target those who are familiar with building sites as the used to be built in the 1990's - that is, using HTML tables for layout purposes. Most CSS advocates decry this approach now - some very vociferously. Those sort of tutorials spend some time comparing the table approach versus using CSS, rather than just teaching the essentials.
It's also worth bearing in mind there are really two aspects to CSS - the styling side, applying font and colour for instance, and the positioning side (sometimes called CSS-P) which deals with the more involved details of overall page layout.
BrainJar A well explained mix of the theory and practice of CSS.
CSSJuice A fine site, of particular note is this round up (ha!) of techniques for creating rounded corners using CSS.
CSSPlay
Cutting edge, experimental even, this is a terrific resource put together by Stu Nicholls. Prepare to be amazed!
HTML.net A good starting point for beginners to CSS.
MaxDesign Sound tutorials, and plenty of practical real world examples as well.
PageResource It doesn't really get into postitional aspects of CSS, but it's a decent place to start.
W3Schools Well organized, the simple layout makes it easy to find what you're looking for.