Hi there and welcome to the world of CSS. Once you get the hang of the basics, I think you are really going to love it, and we are here to help.
Since you are using Firefox, I would recommend the Firebug add-on which allows you to identify the CSS selectors for the different elements and parts of your site so that you can then change them. It shows the HTML code on the left and then the CSS code on the right.
Take a little bit of time to look at some of the documentation and watch the introduction to Firebug video and that will get you on your way with the basics.
You are currently using a very good responsive width themes, and widening it is quite easy as there is only one thing to change, the maximum width in the #page rule. Go to the CSS edit page and delete the informational text and paste in the following and change the 1000px value as desired.
This will widen the theme and the main content area and the sidebar area will widen in proportion. If you want to narrow the sidebar or change the proportions of the main content and sidebar, that can be done pretty easily as well.
Work a little with Firebug to get the general hang of it and when you have questions, we can help get you going in the right direction. I'll offer the following support documents in the event you have not seen them, which can help with the basics.