In my opinion, you should read specified books about CSS&XHTML (you can find alot on internet, or at least in bookstore) and practice. Practice makes perfect. Start with simple things and go forward step by step. And DO NOT start learning from advanced techniques - you'll have more trouble.
As for books, when you'll be more experienced in CSS, I recommend you to read O'Reilly - CSS Cookbook. This book helped me to think different about CSS and after reading this book, I understood, how little I knew.