File Sharing Between two macs?

I'm having trouble sharing files between my two Macs. I've got 3 different Macs. Two of them are Power Mac G5s and the other is the new Black Cylinder Mac Pro. I can see the Mac Pro on one of my Power Macs but not the other. I'm new to Mac so I don't really understand how to set up file sharing, which I think is my issue. Can someone help me figure out how to file share between these computers? Please let me know if I need to provide any other information and I'll do my best. Thanks!