Well im gonna take it that you have broadband right? Well the only way to share it, thats if u have only one ip, is to have a router, or have the NIC that comes out of ur modem (dsl, cabel) hocked up to one of the computers that you will have to keep on 24/7 (if u wish too). So that means you will have 2 NICs in one computer. Set up a proxy on that computer for security.
This is just a very basic idea of what I think u need to do, if u need the details then please post back.