phpbb used to be installable via Fantastico in the CPanel, but is not available that way anymore. If you wanted to try it, installing it manually would be the way to go, and shouldn't be too complicated. You should be able to find step-by-step instructions on the phpbb site. Typically, those instructions would involve:
Creating a database and a user for that database (using CPanel).
Giving that user the necessary permissions for that database (again using CPanel).
Uploading and extracting the phpbb files.
Running any necessary setup scripts, and possibly making some small modifications to one of the phpbb files.
But really, don't do any of these steps without first going to the phpbb site and looking over their installation instructions. And if you get stuck, I'm sure some folks around here can get you over the bumps.