CSS, how does it work?

I suck at CSS and I dunno what to do. I know the syntax, selectors etc. and have no problem coming up with a design but I srsly cannot make it work. All I want is a basic two column layout with a top and bottom and can't figure this out for shit. Does anyone have a resource for actually learning this stuff well? I mean how to actually do an effective layout, not just "hurr durr, use property a to do whatever"