All the computers need to share a network. That is, they all must have wireless cards and logon to the same base station.
Each computer must have its own IP address, which is usually assigned automatically by the base station. You can check this address in System Preferences --> Network --> Airport. It'll be four numbers separated by dots, similar to 18.104.22.168.
Next, all computers need to have the game installed and running that you wish to play.
From there, it should be easy. Details vary a bit depending on what game you're playing.