My CSS basic know-how has been gathered much in thanks to a Mac program called CSS Edit, which is a realtime WYSIWYG CSS editor. Meaning, I can change values and the webpage’s CSS pimped elements update instantaneously.
My recommendation is to skip the step of reading up on whatever rules does what, and just test things out with a WYSIWYG editor. Eventually, you’ll learn the basic stuff from there.
But.
For further (or prior?) reading, you should know the basics of how to include CSS into your document. As well as CSS specificity, nesting and traversing, etc. A good place to start out is the sitepoint references on both HTML and CSS.
You may also benefit a lot from buying their respective books, “The Ultimate HTML Reference” and “The Ultimate CSS Reference”.
As a last hint (that has been suggested before in this thread, is to use firebug in Firefox along with sitepoint’s firebug-plugin called firescope which works as a quick search/lookup to their references.)