'Float' causing sidebar to mess up

Hey everyone. I am new to CSS and have been making some very simple sites to improve myself. For my most recent messabout, I’ve been working with this expandable sidebar menu - http://www.dynamicdrive.com/dynamicindex1/navigate1.htm

It all works fine, but I am wanting to add content to the right of the sidebar - in the large blank area, and when I set the sidebar to float:left, it completely screws up the entire layout. As seen here:

If I am going about this all wrong, or you have any suggestions on what I might do to correct the issue, they would be very much appreciated!

Thanks in advance

Brendan