It's not really hard at all. I believe there are step by step instructions included with phpBB. Just make sure you have the mysqld service running:
# chkconfig mysqld on
# service start mysqld
Also if you need to create a database and user ahead of time and you are having a hard time doing it from the command line you might consider using
webmin. I actually use webmin for some MySQL and PostgreSQL stuff but that is about the only thing I use webmin for.
Oh and make sure you have "php-mysql" installed in addition to mysql, apache and php. And if you are running on RH8 with Apache 2.0.4x then you'll need to perform a slight modification on the phpBB source to work around a caching issue. I can help you with that part after you get your database and everything else set up.