All Powermacs have ethernet built-in as standard. All you need is an ethernet hub - about £30-£40 will do the job... and cabling - available from almost any computer shop.
To connect more than one computer to the internet you will need a router. For example, this is linked directly to the Broadband and appears to the outside world as one device. All the technicality involived in the Macs attached to the router are taken care of by the router itself in the form of NAT 'Network Address Translation' tables.
For connectivity to the internet all the Macs must be assigned different Network Addresses 192.168.0.? or setup as DHCP - this will allocate the addresses automatically. Just go to the control panel, select TCP/IP and set up DHCP from there.
To actually share a folder on a Mac you need to do the following:- a) control panel, file sharing, START FILE SHARING b) control panel, file sharing, users, ALLOW GUESTS TO CONNECT c) select a folder or drive, go to get info and set up sharing in the dialog box. Then go to another Mac on the network, select apple, chooser, click on Appleshare and you 'should' see the shared folder/drive on the other machine.
Macs have virtually no viruses in contrast to PC's.
The above is fairly basic - and i'm not sat in front of my Mac so it is off the top of my head - but it should point you in the right direction.
Sorry if a couple of points are not that accurate!
Regards
Duncan