Buy a SOHO router, small office home office. It will include either a 4 or 5 port integrated switch. unplug power from the modem, connect network cable from modem to router. Turn on modem and let it initialize. Turn on router and let it initialize. Plug in both computers to the switch ports, or connect wireless. That's it.
Be sure to go into the router setup and set security for the wireless and change the default login password.