If you choose HTML or another Web page File type, then you'll be presented with a large number of pre-built templates. Dreamweaver CS3 makes it easy to create fixed, elastic, and liquid width layouts in all the common formats. Just choose the layout you'd like, verify it's correct in the preview window, and click Create.

If you'd like to use a layout you've designed yourself or a free Web template from somewhere else, then you should choose "<none>" in the layout column.