Get yourself a router, Netgear, Linksys, etc (if you use DSL/Cable) and two CAT5 cables. Your cable/DSL modem plugs into the WAN port, and the two computers plug into their own LAN port on the router. Configure each computer for ethernet networking.
It's pretty each, router documentation gives you a fair idea on how to connect the computers, as for configuration of software, Networking in System Prefs is where you go, set it up for DHCP.