Hi, a lot of things can cause this, usually a missing <div> tag or a missing closing “</div>” it will be easier to get help with css and html if you can share a link to your site.
Regards
Yes I know there’s many issues and fixing them one by one. Starting with the category float….http://www.zarchiwebdesign.com/blog/
You have 404 errors too. It seems that you have edited the Twenty Ten theme incorrectly and that causes the errors. Ideally you should have created a child theme and made your custom changes on the child theme. I suggest to revert to Twenty Eleven theme, correct the problems, and then revert to a child theme of your current theme.