A grid typically will have a set width for a website, and then columns that include margins built into them. you can apply the grid like so:
What this does is create a generic “content” and “sidebar”. A grid the width of 8 columns, another the width of 12, and a total of 12 columns wide. This markup is based on http://960.gs/ by Nathan Smith, I recommend checking it out. It can help a lot if you are in a hurry!