Get books on CSS, or books about XHTML and CSS working together, because some books teach "Microsoft CSS", which involves changing the colour of your scrollbars (it's not valid CSS, but changing the colour and style of form elements is).
Or you can learn CSS how I learnt it...by picking up bits and pieces from here and there (not recommended 🙁)
As Goldbug's signature indicates, the road to hell is paved with nested tables and spacer gifs. Nested tables are okay as long they are being used to display tabular data, and not to lay out a website. Spacer gifs are simply unacceptable (though thankfully they are not too common).
Search for CSS tutorials on the web, or search on CSS layouts. You'll learn a lot just by looking around....