AC blows hot air when turned on, but compressor is working.

It sounds like you have temperature blend door that is not moving. This door is inside the HVAC box under the dash and it directs air over the heater core/evaporator core in different ratios to give you the temperature you're wanting. The most common failure is the blend door actuator, but it can also be the temperature control switch or the blend door itself. The first step should be to isolate which of these components has failed so that you are not replacing parts needlessly. The temperature blend door actuator and temperature blend door both involve removing the dash board to gain access and will be labor intensive jobs. The temperature control switch is much easier to get to but the part will be more expensive. There is not likely going to be a "cheap" way to repair this problem, unfortunately.