CSS isn't big. PHP, javascript and other server side languages are alot bigger.

I recommend learning xHTML+CSS then if you want to develop your coding even more, PHP+MySql, then even move onto javascript then ajax.

It all depends on how far you want to go.

I recommend using tizag.com for the basics and understanding, or ordering some of sitepoints books, such as:

Art and science of Javasript
Html and CSS
The PHP Anthology