With CSS, the important thing to learn first is that there are three ways to apply CSS to elements on your page. The first and simplest is to add the style as part of the specific tag.
Inline style Example:CODE
<a href="http://example.com" style="color: pink;">example</a>
This is the easiest to start and learn with, but is the most difficult to update later, as you have to find and update each and every instance of it.
The second way to implement CSS is to use Classes and style information within the page, but in a separate place usually near the top.
Internal style Example:CODE
<!-- in the top -->
<style type="text/css">.sitemessage {color: pink;}</style>
<!-- later on down the page -->
<p class="sitemessage">Welcome!</p>
This is a little easier to change because you would only have to edit the one location and the rest of the page would follow. The problem here comes when you start to have lots and LOTS of style code within the page, it overshadows the actual page content, causing it to download slower and watering down your search engine results (search engines see the style content as text on the page).
The third way to implement CSS is to use separate CSS files, such as 'style.css' or 'custom.css', that load congruently with the page and apply their rules to the page. This is done by linking these text files to the page in the <head /> with the following code.
External style Example:CODE
<link rel="stylesheet" type="text/css" href="style.css">
<link rel="stylesheet" type="text/css" href="custom.css">
If you take a look at the full source or your store, you will see that our software automatically adds these two files to every page. The first one listed - style.css - is updated every time you make a change and click save within your admin panel Site Layout menu. The second file - custom.css - can be downloaded from the file manager and updated manually. The browser loads and applies the first file first, but if there are any rules in the second file they apply in addition to OR override previous rules in the first file with the same class.
Once you become familiar with these methods, you can then move on to using ID selectors - <div id="navigation"> - and other more advanced methods to really make your site pop. Here (again?) is my favorite CSS tutorial site -
http://www.w3schools.com/css/ - where a large portion of my knowledge comes from. I hope this helps.