beside w3 school, visit htmldog.com and buy the book HTML Dog: The Best Practice Guide to XHTML and CSS by Patrick Griffiths
i have a lot of notes here: http://ryancanfield.com/category/css/ but other than that i'd use w3schools.com it's your best bet good luck!
You do know that HTML and XHTML are the same other than the syntax, right? And if you've been serving your "XHTML" as text/html then you haven't used XHTML.
Someone likes to get technical Let's start the whole "you can't code" thing too If I mention the word markup, people are like wtf no I want code...
www.w3schools.com/css/default.asp is the best website to learn about CSS and i recommend a book called CSS Mastery by Andy Budd http://www.cssmastery.com/
Yup, w3schools.com is your best resource. They have to the point tutorials with no BS, and its the best place to start, although I wouldn't recommend it for advanced stuff.
I found out about www.html.net, it has an HTML and CSS tutorial - with such little CSS knowledge you'd be able to start making some good templates, good luck .