If you put a div around the main content area, then you can restrict the width of it (as well as position it where you want it to display), and you can then include a side bar.
Alternatively just place the sidebar within the main content div and then use css to make it float right (or left) and the content should flow around it.
Simpler solution would be to place adverts under the content area – there’ll be less coding for you this way.