First, save a copy of your page with another file name. Then delete everything that you _don't_ want to be on every page--image, texts, etc. that belongs on only that particular page rather than all pages. Next, add an editable region (Insert...Template Objects...Editable Region) anyplace where you will want content of some sort on each page, even though it won't be the same on every page. (This last step is optional, but it makes it easier to add the content consistently later.)
Once you have the page down to exactly the things you wish to use on every page use File... Save As Template. Then, to create another page with the same elements to which you can add specific content, use File ... New ... Create From Template and select the template you just saved. You should also follow this procedure to recreate your original page (make a new page from the template with the shared elements, then cut and page the specific content from the original) so that it will be based on the new template. Otherwise, changes made to your template won't affect the original page.
Once you have your template, you can make changes to every page based on that template automatically. If you decide, for example, that every page needs a logo in the upper right, add it to the template. Then, when you save that template, it will ask if you want to update every page based on that template, and you will say yes.
[Instructions were based on CS4 version of Dreamweaver; I can't promise that other versions wouldn't differ a bit.]