It is sort of a PHP and XHTML question. So here is how you can go about it. Create your pages with the following format or comparable. The following would be index.php
Save your header file as .inc
<?php include("header.inc"); ?>
<?php include("nav.inc"); ?>
<?php include("footer.inc"); ?>
Your footer file as .inc
Then save your navigation XHTML in the following nav.inc. All should work out for you. If you need more help just ask. All of your pages must be renamed to end with .php or this will not work.