[Theme: Reddle] Changing column width

I would like to change the width of my columns, but I do not know anything about CSS.

I’ve been looking around and tried different things without results. I would like the main column to be wither and the side bar to be narrow.

I have found a blog that is a good example of it: http://maedchenmitherz.de/
And this is my blog: http://www.siropedefresa.com/