I'm sure you will find everything you need at the link that Allen provided.
Include is a pretty simple implementation, assuming that your server is set properly.
Normally if the file you want to include is in the same directory you would use the following tag, though I've notice PHP is very forgiving.
include ("filename");
if you want to include a file that is in another file, you will need to call to the file like this.
include ("/path/to/your/filename");
Now some servers/accounts are set up with a defined location where the include files will be located.
To find your include path and many other php variables, create a file with the following tags:
<?php phpinfo()?>
Good luck and welcome to php
Ron