The server should have included a HP Smart Start CD or DVD. Drop that in the machine and boot off of it. It includes the utilities to configure the RAID array as well as a host of other parts of the server.
look for the documentation. along with it should be an HP configuration manager CD. that should allow you to setup your RAID if you boot from it. this will completely wipe the drives!!
if you do not have this then watch the boot screen, during boot the RAID configuration should come up saying something like PRESS CTRL-S TO CONFIGURE SCSI ADAPTER or something like that.