as that 500GB SATA hard drive would be blank when you purchased it.
Take the OS X installation DVD, put it in the optical drive and boot the G5 from it by holding down the "C" key at the chime. Keep it down until you see the spinning gear.
Once booted, choose utilities from the Installer menu and format (initialize) your new drive using Disk Utility.
Once initialized, reboot, again holding down the "C" key and proceed with installation of OS X onto that 500GB drive.
Hopefully your cry for help does not mean that you do not have any OS X installation disks.
i just got a 500gb sata hd and i have no os on this g5 power mac. any help would be greatly appreciated.