Learn SSI

SSI stands for Server Side Included. Make sure that your host supports SSI. Hosts like Freewebs, & Awardspace support it. If you host doesn't support it, then you have to switch. Change the ending of all your pages to .shtml. For example pokemon.shtml. Now make two new pages. Name the first one top.txt. Then put the first part of your site's layout into it. This is the beginning of the coding, right down to where the content would start. Name the second page bottom.txt. Put all of the HTML after the content into it. You are now done. On your pages You simply put
<!--#include virtual="top.txt"-->, then your content, then you put
<!--#include virtual="bottom.txt"-->. If you change anything on your bottom.txt, or your top.txt, then the enitre site will change.