Most likely if it is blowing hot air, and everything including compressor seems to be working, it needs a charge of freon. You can pick up a bottle of freon at Auto Zone, they sell bottle of freon and even has a guage on it. If you drive the car in there and pop the hood, they will show you where you hook it up. It even tells you step by step on the can how to do it. But just to give you a recap of how you do it: You first have Auto Zone tell you the connection on your car where to tap the hose off the can to. You start engine, turn on the AC at full speed, and shake can real good, hook hose up to the connection that Auto Zone showed you, and turn can ( light shake) while its hooked up from a 12 noon position to a 3 O'clock position back and forth while depressing can every 10 seconds. Pretty easy to charge your own AC.
Now realize that the reason your car is just blowing warm air, is most likely it is low on freon, but don't forget it is very possible it is low or out, is because of a leak. DO NOT get suckered by these auto shops saying, you have a leak and we have to do this and that and all that. Cause they will charge you a lot of money to check it. First step is to try and charge system first and see how it works. The freon can at Auto Zone runs around 30 bucks, worth the cost to see if that solves it. Otherwise a shop could cost you a few hundred for the same results of just charging like you did for 30 bucks. If this does not work, then you can have them put system under pressure to see if a major leak in line somewhere. Thanks Mike