I'd suggest that you go with one of the mainstream/common Linux server distros: either RHEL (for which you can use CentOS, which is essentially the same, minus the RedHat-copyrighted bits) or Ubuntu Server.
Either of these can be configured to use a GUI. I'd actually pick RHEL/CentOS over Ubuntu, and during the install (which is graphical), you can select to install a web+database server along with a Desktop (GUI). The installation is fairly straightforward; the most complex part is arguably the partitioning, although you can use the guided partitioner to just use all free space on the disk. Partitioning isn't something that's linux-centric, although the partition scheme for Linux is perhaps a bit more complex than what'd you would expect coming from a Windows world (dedicated swap device, LVM to virtualize the partitions, etc..). If you use the guided "do it for me" option, you can avoid getting your hands wet with this complexity.
The primary reason I'd suggest going mainstream is that the support will be there. If you choose some OS that no one really uses, you'll be hard-pressed to find distro-centric documentation for it. If you go with Ubuntu or RedHat, you can use Google to get through any obstacles you may find. There are plenty of tutorials available when you google for a simple [do this task] on [this distribution]. For example:
While you could probably use this documentation to complete a task on another distro, it's helpful to have a tutorial for the specific OS you're using; all the commands will be the same, and any dependency problems, etc... will all be accounted for.
Additionally, should you decide that you want to learn more and play around, having something mainstream installed means that your learning experience will be directly relevant to anything you want to do down the line.
As an alternative, you could go with a pre-built phpBB appliance. http://www.turnkeylinux.org/phpbb is a single ISO or VDK that is built on Ubuntu Server and comes pre-configured with phpBB (they have many other applications available as well - highly recommended!). It'll ask you a few questions during the install, and once complete, you'll boot up into a fully-functional Linux server with phpBB already running.