Create a complete web page with your nav buttons, banner, footer, and some sample content. Highlight your sample content (the stuff that will be different on each page) and go special->template->new editable region. You can make as many editable regions as you need and with the template regions tab you can give each range a name that makes more sense to you.
Next we have to save the page as a template. This option is often hard to find. Near the top right corner of your pages window (just below the close window icon) click the right facing arrow and select save as... template.
Each time you create a new page create it from the template you just made (i usually right click in my web site pane and go New/Create from template). On new pages you'll only be able to edit the editable regions you set earlier, but at any time you can reopen the template and any changes made to the non-editable areas (add buttons, change links, even complete redesigns) will update all pages created from the template.
BTW: All this, with more detail and options, is in the help files.
