Be aware that in the coming years what you learn here may change.
XHTML is currently what most people prefer (as opposed to HTML v4), however, the next implementation of XHTML (commonly known as XHTML2) is going to be a subset of HTML v5 rather than an extension to XHTML v1.
Confusing eh?
In all truth, the basics of the syntax are pretty much the same. You say you know basic HTML, there's very little difference except that you must close all tags in XHTML (like <img /> rather than just <img>) and of course must use a different DOCTTYPE. There's further fancy things you can do syntatically, but they're really not useful for coding websites, as you can use both CSS and ECMAScript (ie Javascript) in both
HTML.
The site Nathan linked is propably the best starting place.