How to Change the Margin-Padding of the Main Content

Hi i want to change the white background, where i put my content, to make it more big because when i put ads the ads are bigger that the background and they are being cut(the half ad is appeared).I don’t know what change to make in the editor of my theme.Can anybody help me? I can send you the style.css. My site is this one:
You will understand what i mean.

Thanks, I appreciate your time

John