Adding border between posts and sidebar

Hey,
So I am completely new to wordpress, and any sort of website developement for that matter. I have been learning as much as I can for the website I have been putting together, http://www.bestupnext.com, but recently I’ve been stuck on one specific issue. I’m currently using the default wordpress twenty eleven theme,and id like to put a border between the content and sidebar.I’ve looked through almost every forum but i was unable to figure it out. If someone would be able to help me out and explain it to me in the simplest of css terms that would be amazing,
Thank you so much!