My personal favorite free forum script is phpBB, but you'll need to have mySQL on your server to run it, and if you're not familiar with the database structure of your server, you'll want to have a MySQL administration console... it makes it a lot easier to manage your databases (free as well, but your webhost may charge for that service).
If you haven't got much time, and want to see what the new phpbb3 can offer, just try it out on this host :
create forum
Hope this can help!