Can you describe your network in better detail??
Is your internet dialup or broadband?
If you have broadband, you can get a router, and that would go between the computers and the broadband modem. Modem plugs into the WAN port on the router, and PC's plug into the remaining ports.
If you are using DIALUP modem, and want to hook the PC's together, you will have to use a CROSSOVER ethernet cable to connect them. You will also have to enable INTERNET CONNECTION SHARING.