I assume that your cable modem is connected by ethernet?
I both computers have Airport you can set the Sharing Preferences to allow other computers connected to your computer to use the connection.
My home system is the other way around in that my laptop connnects to the network by Airport, so sharing is set to allow other computers connected to the laptop by ethernet to share the Airport connection.
Have a look at the options in the System Preferences/ Sharing bit. Just need to tick the right boxes to enable sharing the connection.
I fyou only have ethernet connections you an just plug the cables into the back of the router/modem. If you don't have a router, but just a modem with a single ethernet socket you will probably need to buy a switch/hub so that several ethernet cables can link up to form the network.