You need to use your OS Install DVD or Install/Restore DVD that came with the machine to format the drive. To get the drive to open at power-up, hold the left mouse button down as you power up and the machine should open the tray. Insert the disc. If you have the flashing question-mark/folder icon, the machine should eventually recognize the disc and boot from it. If not, you will need to power the machine down and bring it back up holding the 'C' key down on the keyboard.
When you get the installer running, *do not proceed farther than the language choice*. Select from the Utilities menu "Disk Utility" which will bring up Disk Utility and you should see the hardware indication for the internal hard drive in the drives pane in the Disk Utility window. Select the Hardware description and icon in that window, not any volume indication indented and under it.
Look at the bottom of the Disk Utility window at the information and you should see an entry for Partition Scheme which for a G5 should be "Apple Partition Map" but for your PC formatted disk probably will not. We'll deal with that in a moment. (You might see "GUID Partition Table" or "Master Boot Record" which may require an extra step below.)
Now, click the "Erase" button which will bring you to the place you start the formatting process. Make sure that the Volume Format selection is "Mac OS Extended (journaled)" and then name the main volume if you want, or leave it at Untitled for the moment (it is better to name it at this point).
If the drive is a new drive, then you can probably ignore security options, as they involve writing zeros or various data patterns on the drive and take a seriously long time to complete. Click the "Erase" button and you will shortly see the format complete and the main volume on that disk should mount. At this point, you should be seeing "Apple Partition Map" as the Partition Scheme entry. If not, there is a little more work to do.
To get yourself to Apple Partition Map, you need to go to the "Partition" tab, click the "Options" button and make sure that "Apple Partition Map" is selected, click OK there, make sure format is "MacOS Extended (journaled)" and name the volume, then click the "Partition" button.
Once all that is done, then you should be able to quit Disk Utility and commence the OS install.
Some added links:
http://support.apple.com/kb/TS1600
Message was edited by: old comm guy