The AC is not cooling

Most of the time the problem is low coolant in the airconditioning system. When the pressure is too low, the compressor won't even come on (there is a clutch on the compressor, and you can hear a click when it turns on or off). There are test kits available at automotive stores which come with instructions and a pressure gauge. Sometimes the kits also come with a bottle of coolant. You need to make sure the type of coolant is compatible with your car's system. Keep in mind that the reason the coolant is low is because there is a leak somewhere. If it is a really slow leak, you could get a season of cooling out of it after recharging. However, if the coolant leak is more significant (due to a bad seal or a puncture), you may only have a few days of cool air, and your money and effort will have been wasted. A competent mechanic can test you system for you and determine the source of the leak. Cost is usually between $100-200.