The easiest way would be to backup you site, then delete the files in the public_html folder, and upload SMF using FTP. Once everything has uploaded you need to go to your website, and install the forums.
There done.
Note - This will erase your current site and replace it with SMF (Is this want you wanted?)
If that isn't what you wanted you could include the forums in your current homepage. You would still need to upload SMF and install it but this time put it in a folder called 'smf' or 'forums'. Then put this in your homepage (It must be called index.php) where you wnat the Forums to appear.
<?php
include(/smf/index.php)
?>
If something goes wrong you can always upload the backup you took.