Clear: both
It defines what can be shown to the sides of that element. If you clear left, nothing can be shown to the left. If you clear right, nothing can be to the right of that div.
Here.... if you look at my stagnant site (yeah, not much on there but I have no time! lol) Patterson Web Services the entire design is CSS. You can view my CSS code and see how a clear works. You can also read up on clearing items over at the w3schools.com site.
CSS is easy to learn; however, that's not to say it doesn't take time. You need to have a will to learn, and a want to learn it. It's tough to code because you have to code on an almost per-browser basis. A tip would be to start coding for Safari, Konquerer, and Firefox first. Then use "hacks" and work-arounds for IE. If you google for IE hacks and CSS, you'll get plenty of hits.
CSS can get really in depth, or it can just be something simple. CSS is a set of rules with definitions. You set up "rules" by defining what elements (and their classes/ids) are to be "styled". The definition outlines how the element will look. You can read more on the w3schools.com site. If you're interested in getting into css, I'd say you spend some time just learning about the basics (like styling links, forms, and the like). Then you work on moving away from a table layout. The hardest part about CSS is getting the elements to work with eachother in every browser. It's doable, just takes time.
EDIT
Your ads don't show up properly in Firefox on linux (as of 22;51 Feb 17, 2006 EST). The ads show up underneath the left column!!
You should use a clear: right; to solve the issue.
One other way is to float your table & navigation left, and the ads right, then just clear both after the ads.
It also doesn't help that your table takes up 97% of the browsers realestate:
<table width="97%" border="0" align="center">
This isn't even really a CSS quesiton... you're using a table layout.... fix your table and you'll fix the ads....
