Assuming you have Panther or Tiger on the G5; take a look at the Active Directory tool in /Applications/Utilities. You will need help from your system administrator, since he has to tell the domain controller to let you join and then provide information to you to do it.
As for installing printers, that varies wildly depending on what kind of printer. If it's a USB printer, just plug it in, it will probably detect it. If it's a network printer, open Print Center, and take a look.