To further DotComBot's comments (good comment, btw), you need to setup the CSS this way (for external stylesheets, which I highly recommend):
<link href="stylesheetname.css" rel="stylesheet" type="text/css">
The above text needs to go in between your <head> and </head> tags in your HTML document (or php, asp, whatever)
Then, you just need to edit the CSS document (whatever you named it, it needs to end with .css).
Here is a great place to get started learning CSS.
Hope this helps and I didn't step on anyone's toes
