Once you get the car have someone who knows a bit about cars point out the low pressure switch for you, it's usually located on the reciever / dryer. With the AC system on unplug the connector and jumper it with a paperclip, if the compressor clicks on then the electrical part of the ac system is ok and the system is just low or empty of refrigerant.
If this is the case have a vacuum applied to the system and see if it holds, if not then there's a leak that needs to be fixed. If you can't find anyone to apply a vacuum just fill it with about 2 to 2.5 of those heineken keg can shaped refrigerant cans and see if the system stays working. If not then it's leaking.
WARNING. This is not the proper way to repair ac systems but I do it anyways and have done it to 3 of my cars, I don't care. But be warned, it is kinda 1/2 assed. The proper way is to fix all the leaks, replace the reciever dryer, have the system exacuated and flushed and then refill it with the proper ammount of oil and refrigerant that Chevy calls for.