making a div for the very top section register, login?
make a div for the site logo/advertisement?
make a div for navigation menu?
Lucky for you, that’s kind of like asking what section of a car to start washing; front or back? There’s millions of ways to create this layout, and the only "that’s wrong" you’ll hear is if it doesn’t work in all browsers.
That being said, here’s my basic thoughts:
Create and style divs for all of the biggest rectangles on the page. (header, leftcontent, rightcontent)
Inside those main divs, create the smaller sections…. repeat until everything is in it’s place.
As you can see, it’s very simple. There are many, many ways you could go about doing this. Just look at your picture, and put it into div’s. Remember, use the HTML for the markup, and rely on your CSS to give your page the "looks".