Install Firefox if you don't have it yet, then get Web Developer from www.chrispederick.com (its author).
In the Web Developer toolbar, the Information > Display Element Information button will show you lots of CSS-relevant information about any element you click on.
The CSS > Edit CSS button will let you try out styling with the classes/ids that the info button shows you, and see the results instantly.
To learn what kinds of changes to make, you should know something about CSS - it will be useful to you in any other web design work you ever do as well. Look at www.w3schools.com, and look at your stylesheet too - you can learn some things about styling from what is already there.