Be aware of the signs that there is a problem with your air conditioner. No cool air at all or a lack of cool air are the main signs that there is something wrong with your air conditioner.
Know the parts of a car's air conditioner. Basically, there are 5 major components to the air conditioning system: the compressor, the refrigerant, the condenser, the expansion valve and the dryer or evaporator.
Learn what each part of the air conditioner does. The compressor is the power unit of the air conditioner and it pumps out the refrigerant, which is a chemical gas that condenses at a low temperature. The condenser is what is used to change the refrigerant to the liquid form, while the dryer or evaporator turns that liquid into cold air. The expansion valve removes any pressure from the condensed refrigerant and moves it into the evaporator.
Check to see if the refrigerant is leaking. Use a fluorescent dye that you can purchase at an auto parts store like Autozone or NAPA.
Follow the directions of that particular dye. Generally, you will add the dye through the low pressure A/C recharge port located on the car's A/C canister.
Check for leaks using a U/V light. If the refrigerant is leaking, many auto supply stores sell refill kits with a leak sealer that you can do at home.
Try to determine if there is something else wrong. You can check to see if you have bad fuses, broken wires, a broken fan belt which is keeping the pump from turning or an incomplete seal on the compressor. These parts are easy to replace if they are the source of the problem.
Try your air conditioner after each repair you make. If you exhaust your ability to replace parts, or if you can't find anything wrong and the air conditioner still doesn't work, it may be time to take it to a professional.