Is your site currently on a WEBHOST? If so, more than likely they already have PHP and Apache and Linux or some other OS installed.
All you need to do is ask them about MySQL and if it is available.
You will then need to download the PHPNuke pack from
http://phpnuke.org
Follow the install instructions to get it running on your site.
As for the phpBB Forums that are integrated into Nuke, you can find it here
http://bbtonuke.sourceforge.net
mikem