Problems using and removing sidebar

I seem to have a few unused sidebars on some of my site’s pages, and I’m not sure how to edit/remove them.

My site is: slsc.fiu.edu

On the first 3 main pages: About, News, and Events
Each of those 3 pages has a sidebar on the right side that isn’t being used for anything, but I can’t find how/where to edit or remove them. A couple of my pages are using a sidebar for a menu, and although I want to keep them there, I also don’t know how/where to edit those. I would like to be able to add a sidebar to my homepage, with an embedded Twitter feed.

Thanks!