First rule of making layouts work: use a valid DOCTYPE to ensure the browser is not in quirks mode.
e.g.
code:
<!DOCTYPE html>
<html>
...
</html>
That's the proposed HTML5 DOCTYPE, which also works for HTML4 and XHTML1, and is what I use.
Then I'd say make sure you understand the (differences between the) following 14 CSS items, very roughly grouped...
Types: display, position;
Dimensions: width, height;
Edges: margin, outline, border-width, padding;
Positions: top, left, right, bottom;
Floating: float, clear;
Think that's everything, but possibly missed something.
Knowing what they are and how they work makes layouts much easier (and should help you understand other tutorials).
EDITED: 12 Feb 2009 16:08 by BOUGHTONP