As LyteNight said, you can download the latest stable release of phpBB package. Upload it to your directory. Extract the package in the directory created for your sub-domain. (The package may also create directory when you extract it, check the package structure and extract the files as per your need). Create the MySQL Database in your cPanel, Add the user to it with all necessary privileges and point your browser to the phpBB Forum installation script and follow the instructions on the page.
Or else, you can simply use cPanel's Fantastico
All the best.