My advice is to open a NEW HTML page. Build it with the header, menus, footer and other page elements that will remain constant throughout your site.
Run the code through validation checkers to ensure it is error free. Fix any reported errors.
Link the page to your external (CSS) stylesheet.
Test this prototype page in all major browsers. When you are completely satisfied with the look and performance of your prototype page, hit File > Save As Template.
Now, Add some editable regions for content that will change from page to page. Save again. Close.
Open a NEW Page from Template. It should look exactly like your Main.DWT except that you won't be able to edit anything that isn't inside an Editable Region.
Guidance on when to use DW Templates, Library Items and SSIs -
More on DWTemplates -