What's wrong with the A/C in my car?

I have a 1992 Olds Cutl Ciera. When I turn on the vents it will only blow on defrost, no matter which position I put the knob. Hot or cold. When I have it on A/C it does not blow cold from the defroster either. I would like to get it fixed, but would like a little more info beforehand. Could it be the switch??? Any help is much appreciated, because it's HOT in Florida. :)