I am not going to do a file on style.css, because I guess everybody knows how a style.css file looks like. Anyways, what is particular for a wordpress theme is that you can add a code at the top of your style.css to describe your theme. This will show up in your dashboard, in the themes section.
The code is the following:
So you add this to your style.css. The rest of the style is up to you. You choose your div, you choose how you customize your layouts and what tags you use. It’s exactly like php includes or html layouts. If you want me to write some css tutorials, you just have to ask me on this page.