Creating box for margins

I purchased Dreamweaver CS3 The Missing Manual and I'm learning the basics of
this program. When I went through the steps in the book the first time I very
easily created a margin around my page so that there is a white area where my
page is and my background image in the margins. This worked fine on the
tutorial program, but whenever I try to make it on my site I am just able to
come up with a small box in the middle of my page, which is of no use to me,
obviously. I went through the book again and did all of the steps and the same
thing is happening. Does anyone know if there is an option that I have
overlooked or something that I have checked that I need to uncheck? Thanks.