I am trying to reinstall the serial number on my OS X Server in order for me to be able to use my Mac. I have a G4 desktop if this helps. The serial number is not on the unit unless I am looking in the wrong place. I am not to familiar with Mac's help me

I was given this PowerMac G4 with Mac OS X Server. I am not familiar with Mac but I can follow instructions well if they are detailed. I am at the begining where I have to enter the serial number and I don't have one. I have tried looking all over the system and all the serial numbers that are on it doesn' work. Either they are too long, not long enough, and or the number/letter sequence aren't correct to enter in. Can someone Help me PLEASE. i would really like to get this system up and running.