Get Firefox and the Firefox Web Developer plugin. With it you can experiment with CSS right in the browser and see almost instantly the results of what you are doing. It's a great way to learn. Your changes with Web Developer vanish as soon as you go to another page, so you don't have to worry about messing up anything. Once you've made changes you want to keep, copy them and put them into your theme's css.
To find the right class/id to edit, do a view > source of the page and search for text that appears in that area. Look for what classes and ids are associated with that area. Then, search for the same classes and divs in the CSS. That usually does it, though sometimes you have to take into consideration that the CSS for plugin comes with the plugin.
I'd start with changing background colors for divs, to make it easier to see how the page is structured.
Like anything else, it's not as confusing after you have your bearings.