New set of o-rings for the keg
A length of CO2 line (3-4 feet) with the proper disconnect to fit over the gas post (the gas and liquid are diffrent disconnect fittings, its easy to see on a pin lock, but harder to see on a ball lock. Ball locks are usualy grey for gas and black for liquid.)
A regulator that has adjustable pressure and a low pressure gauge.
A picnic tap with the proper disconnect to fit over the liquid post - ball lock or pin lock. These are usually 3 feet long, but if they have anything longer it would be best to get something closer to 6 or 7 feet.
Thats the bare minimum i think. I would read the FAQ in the sticky, the first 2 sections will be especially helpful. The more you understand about the relationship between the CO2 and the beer, the easier it is to avoid problems and quickly diagnose the ones you do run into.