Basically you would buy yourself a router. It acts as a gateway and allows internet connection for all the computers connected to it. This is how you would connect your stuff....
Modem ---> Router ---> Other computers
By the way if you don't want to mess around with a bunch of wires you can get a wireless router. Once you got the router just buy yourself wireless network cards to put in all your PC's that you want on the network and the internet.