It's a good idea to familiarize yourself with the parts of a blog before attempting to edit the HTML of your template. I have color coded the different parts to make them easier to see.
Orange- This is the Body. It contains everything you see within the Blogger window, including the blue Blogger navigation bar at the top.
Blue- This is the Outer Wrapper. It contains all of the sections of your blog. Posts, sidebars, all of it fits in here.
Red- This is the Header Wrapper. It contains your header.
Green- This is the Main Wrapper. It is most often the biggest column. It contains your posts.
Yellow- This is a Sidebar Wrapper. If you have a three column template, you will have another one of these, most likely on the other side of the Main Wrapper.