Well, let's face it. It can't all be theory. At some point we need to get down to brass tacks, and that means examining how CSS is used to achieve the design you want. The first step is to prepare your workspace.
You're going to need a folder to hold your website, and a hosting service where your site will eventually live. Make sure you've got your web site files in a folder that's easy for you to get to later. Get into the habit of creating top-level folders to hold the files you'll refer to throughout your site. For example, your folder structure could look like this:
If you're not familiar with root relative links, they're links to files within your site that start with a leading slash. "/css/main_template.css" will point to that css template no matter what page you put it on anywhere in your site ragardless of what folder that page is in. It's extremely important that you use root relative links to attach your CSS files so that you won't have to change that XHTML from page to page.