First, make sure you are running Firefox browser (you’ll be glad you did) and use their Web Dev Add-on tools. Very powerful and incredibly helpful for these things.
Second, play CSS detective with http://codex.wordpress.org/Finding_Your_CSS_Styles and http://codex.wordpress.org/CSS_Troubleshooting
Third, realize that the layout of your site is based upon two things. First, the shape and positioning of things within your styles.css and second, upon the width of whatever you have in the containers on your page. For example, if you have a really long link, longer than the column it is in, it will push the container over and force something, like the sidebar, down. Or if you have a photograph or image that is 600px wide and your content container is only 400px wide. It will shove all kinds of things around to make room, no matter what you set the content container’s width. It has to expand to whatever is in it.
If you did not change any of the positioning originally on your site’s theme, then the odds are that you have a pusher shover on board. If you did move things or change the width or height of your main containers, then you have to go back and fix them. That’s where Firefox web dev tools come into play.
Using the Edit CSS utility, you can change the width of your containers to narrow or widen them and see the changes on the screen as you work. It’s brilliant. USE IT.
Good luck.
