Things to check:
1) Is there any refrigerant (pressure gague)
2) Is the compressor turning on. Watch the compressor hub when the A/C is turned on, it will spin when you first turn it on.
3) If there is refrigerant and the compressor is running, then is there ENOUGH refrigerant. You can watch the eye on the dryer (little megal canister near the front with the peep hole). If you see a pile of bubbles then that's a sign that you might be low. A set of pressure gagues would also be required, you need to check the pressure on the high and low side of the compressor while the A/C is running and see if they are within spec.
Assuming your problem is as simple as being low on refrigerant then you have a leak and you haev to find it. You can find it with a sniffer (expensive, find someone who has one). You can also find it by putting die into the A/C system, then crawling around under the hood with a blacklight. The dye will glow where it is leaking.
Be aware that any leak that is caused by something other than a loose fitting will likely require the A/C shop to empty the system and take that part off to fix/replace it. Before it is refilled with refrigerant it will have to be evacuated (pumped under vacuume for a period of time) to remove any air/moisture.
There you go, that's all I know about A/C, hope this helps. It coule be something simple like a bad compressor clutch, or it could be something bad like a dead compressor. Shouldn't be that hard to diagnose, A/C isn't very complicated.
