Most standard hosts provide the functionality for you to use SSI and it's done by simply adding the following code to your page where you want the included file to show up...
<!--#include virtual="includes/page.html" -->
(where includes/page.html is the path to the included page)
Now if I remember correctly, I had to adjust my .htaccess file on my server in order for the include to work with .html extensions. (I got help from my host) By default I believe you have to save your pages as .shtml and I didn't want to to do that because that would mean the path to all my pages would change.
Just to add a little to the SSI. Generally, web servers will have already enable SSI for .shtml and to enable SSI for other extension (i.e .htm .html), you will need access to .htaccess in your 'public_html' folder, enter the following code in it :-
AddType text/html .htm
AddHandler server-parsed .htm
Options Indexes FollowSymLinks Includes
Above will enable SSI for files with .htm extension.
Just to add a little to the SSI. Generally, web servers will have already enable SSI for .shtml and to enable SSI for other extension (i.e .htm .html), you will need access to .htaccess in your 'public_html' folder, enter the following code in it :-
AddType text/html .htm
AddHandler server-parsed .htm
Options Indexes FollowSymLinks Includes
Above will enable SSI for files with .htm extension.