Test the A/C Compressor Clutch relay, then go from there. Test it before replacing it or things are going to get expensive.
If the relay tests good, unplug the electrical going to the clutch, and hook up a multimeter (making sure the belt or fan won't suck up wires), turn on the car, turn the A/C on, and then verify if there is a voltage cycle or not.
If there is a voltage cycle, then the clutch is malfunctioning. If there is no voltage and the relay tested good, you'll have to check for voltage at the junction box and go from there.