I was having problems with my A/C when I first got my car, but I got it working. You are going to need to give some more info.
Does the A/C get cold when driving on the interstate? (This is a critical question) Does the A/C compressor cycle on and off when the car is idling or when you rev the engine and hold it an elevated RPM (also a critical question)? Is the auxiliary fan working when the A/C is on? Does the engine temp get hot? Is the condensor (in front of the radiator) all gunked up with road crud?
I hate to tell you this, but it if there aren't any mechanical problems with the system, it could be the R-134. These cars have tiny, tiny tube and fin condensors on them. R-134 really requires a much larger flat tube radiator-style condensor in order to get the A/C satisfactorily cool. If we can't nail it down, you may need to take your car somewhere and have it evacuated and recharged with R-12 (about $60 a pound now just for the freon in my area of the country - these cars require a little over 2 pounds of refrigerant).
My car had two problems - the fan clutch was bad, and I think the car had R-134 in it. I had a guy I know who is one of the freezer technicians for a southeastern US grocery store chain evacuate my system and recharge it with R-12 for $30. It works so much better now. But, also keep in mind that these cars really do have pretty poor A/C units in them - they aren't known for getting ice-cold. Even with the R-12 in traffic, the A/C output isn't fabulous (but much better than with R-134).
Replacing the evaporator would be a big, expensive job - the evaporator is the unit in the A/C/heater unit inside the car. The expansion valve could be bad, but lets make sure it isn't something else first because replacing the expansion valve will require evacuating the system and recharging it.
