5 minutes of forum search (or web search) should have given you some results - your question has been answered a few times before;
http://wordpress.org/search/twenty+eleven+sidebar+on+posts?forums=1I suggest the plugin http://wordpress.org/extend/plugins/twenty-eleven-theme-extensions/
Yeah, bullheaded me decided to try doing it on my own. "Directions? I don't need no stinkin' directions!"
I was wrong.