How do I widen the margin in the html of my website with blogger?

My site is the blogspot url on my homepage but so check it out so you can tell me what to do. The problem is the ad to the right does not show all the way meaning I need to widen my margin. How do I do that when I got into the settings. I know how to do most but I want the page to be even, please help. THANKS
How do I widen the margin in the html of my website with blogger?
%26lt;body leftmargin="0" rightmargin="0" topmargin="0"%26gt;