Need help with column widths

Hello,

I had some basic understanding of css but I am having a hard time figuring out how to widen the main content area and push the sidebar further over getting rid of the white space to the right of it.

http://www.ambassadorpitbullrescue.org/

I read the section on custom columns in Montezuma css options but I don't know where exactly to place those codes. I tried using the set options for instance responsive 100% vs Responsive 980px but thats not giving me the affect I'm looking for.

Thanks!