Have you looked to see if the compressor clutch is engaged and is spinning while the AC is on? I would start there. If it is spinning and no cold air is coming out then it is an air flow (vent) problem.
If it is not spinning it is either low on refrigerant, not getting power to the compressor or the belt is not causing the compressor pulley to spin.
For low refrigerant you will need a set of gauges to hook up to the ports. If you have no gauges, to see if it is completely leaked out you can take off one of the caps and just press the valve with a screwdriver (or other cylindrical object) to see if refrigerant shoots out. (Just a quick press will do) I forget the pressure at which the switch will cut the compressor out but you could still have too low a charge which will not allow the compressor to engage.
To check the power, turn on the car and press the ac compressor switch. Pull the wire connection at the compressor and put a multimeter on it. It should have voltage.
Checking the spinning is easy. Look to see if the belt is spinning the compressor while running.
The problem should arise. If all checks out then most likely your refrigerant charge is too low.