Nice server to start with! Old...but features are still what are used today.
Hopefully you got a bunch of hard drives with it, hot swap, a good RAID controller, and a decent amount of RAM.
Learn a few things....
*Depending on the amount of drives, in the BIOS RAID manager, learn how to build RAID 1, RAID 5, RAID 10, etc.
*Install a Windows Server OS direct on it..play around. Download and install HP's management tools...such as the RAID management utility. Pull one of the hot swap drives..see what the RAID manager does, and how to put a fresh drive in and restore the RAID array...all while the server is running!
*Install VMWare ESXi on it...and learn how to install virtual guests and manage them
*Install Microsoft Hyper-V on it...and learn how to install virtual guests and manage them.
Along with virtualization and the ability to install many instances (guests)....on the server guest, promote it to a domain controller. Built another guest with a Windows desktop OS like Win7Pro...and learn how to join the domain you built on the server guest instance (which is now a domain controller)
Learn the hardware, hot swap power supplies? Multiple NICs?