SSI.php is included in SMF. To use it, you have to use these commands on the main page of your website:
<?php include('http://www.example.com/SSI.php?ssi_function=recentTopics'); ?> <?php include('http://www.example.com/SSI.php?ssi_function=topPoster');?> <?php include('http://www.example.com/SSI.php?ssi_function=whosOnline');?>
Please see ssi_examples.php, located on your forum's root directory, for more examples.
For more information on SSI.php, take a look at Category:SSI.
If you are not using a separate webpage for your homepage and people land on your forum index, you can show recent posts on the default forum index as well. In the admin panel -> current theme Number of recent posts to display on board index: