Help w.Positioning Header, Footer, Side Bars, etc

Hello,

I’ve begun editing one of the free themes on the site and will be using a different background/wallpaper. While playing around with it last night, I didn’t notice any positioning code for the files.

The left side bar needs to be in a specific place because there is a cut out/blank spot in the wallpaper just for the left sidebar. How do I position the bars, footer, etc so that it will show up in the right position on my wallpaper?