Do not edit the Twenty Eleven theme. It is the default WordPress 3.2 theme and having access to an unedited version of the theme is vital when dealing with a range of site issues. Create a child theme for your changes. Once your child theme is active, we should be able to help with the customisation.
After you created a child theme or fork (copy onto a different directory). You can edit the style.css file for main page, but do a search for style.css at this site and it’ll return a few pages that will indicates what parameter is for what, etc.
