97 Corolla A/C blows hot air?

I have a 97 Toyota Corolla. When I turn on the a/c it blows out hot air. I can hear the compressor switch on when I hit the a/c button. I bought a can of 134a with a pressure gauge on it. It reads a psi at 150 that is in the red zone and says "may have mechanical problems" so it's not the refrigerant. I really don't want to take it to a shop. What else could it be and how can I fix it myself?