Before you can start experimenting with SSI you'll need to ensure that the server that your site is hosted on supports it. Virtually all professional web hosting companies support SSI and several free ones, among them Hypermart.net and angelfire which im currently using also allow you to employ SSI on your site.
You can implement SSI in two different ways on your server. Most servers by default are configured to search for, and parse SSI calls only in files that end with a .shtml extension. On some servers it is also possible to tweak the settings so that files that end with .htm or .html are parsed. Contact your webhosting company to see if this is possible on your server. If not, you'll have to rename all your pages so that they end with .shtml and change all internal links so that they reference the .shtml pages instead of the ones ending with a .htm or .html extension.
Once you've got all that information sorted out it's time to put an SSI call onto your page. The code that you need to use is:
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What that does is take everything in the something.htm file and dump it into the place where you put the SSI call. This means that the something.htm will NOT have the standard background color and HTML definitions that tell the browser the background and link colors, the title of a page etc. Remember, everything in something.htm will be dumped into the file that calls it -- you don't need to specify Background/link/text colors, Meta Tags and Title Tags in the something.htm file, as long as the file that contains the SSI call has them.
That's it! Upload something.htm and the file that references it to your server and test it out. The next time you need to make a change that affects multiple files, all you have to do is edit something.htm and upload it to your server.