XHTML is just strict HTML. Tags must be in lower case. All tags must be closed (no opening <p> tag with no closing </p> anymore), and the ones that aren't really closed because they define some sort of object like <hr> get a hybrid open/closing tag (<hr /> in this case).
If you know HTML, you pretty much know XHTML but you just can't be lazy anymore with your tags. And there are a couple extra opening statements.
CSS seems like a lot to take in at first, but it's actually really powerful. You can change the look of a whole website by just changing a single CSS file.
Basically the XHTML documents are the actual content. You do the formatting (bold, margins, line thicknesses, fonts, colors, paragraph spacing, etc.) in CSS.
If you can, get Macromedia Dreamweaver. Or at least Homesite. It makes this stuff a lot quicker.