You want to add a forum on your site and you have no idea how? phpBB is what you need.
Important note: This FAQ is for those who already have a web host, because to set up the forum, you must have an FTP server on which upload the folder containing the forum. Installation of a phpBB forum protocol requires PHP4 or higher and a MySQL database on your hosting service.
What is phpBB?
phpBB is a "pre-built" forum both open source and free. It is very customizable and easy to use.
Where can I download phpBB?