Setting up phpBB is fairly simple enough. But this will help set it up using a subdomain (e.g. forum.mydomain.com). You start by adding a record to your DNS. This is usually done through your domain registrar (e.g. GoDaddy.com, NetworkSolutions.com). Add an A record that has forum.mydomain.com as the common name and add the server’s IP address. Once this is done, you must add a virtual host section in the Apache configuration file:
Download the latest version from http://www.phpbb.com. Unzip and upload the folder to /home or whatever you serve your files from and restart apache. Follow the installation instructions and make sure to rename or delete the /install folder under phpBB3
Hope this helps. Take care.