If you are looking at the motor from the front of the car, look to the right of the motor in front of and below the brake fluid well. you will see 2 valves it is the smaller of the two. I knew nothing about putting freon in my uncle told me how. it was simple and didnt see where you could make mistakes as to which to put it in, as the hose only fit on one of them
There is no need to ever change the refrigerant which is R134 and not Freon R12.
Freon is R12. Your 1997 Chevy uses R134 Refrigerant.
On my retrofitted to R134-A the fill valve is underneath the airbox. You have to take it out to get to it.
if u have not done any work on the a\c before and u need to "add coolant" then u more than likely have leak somewhere wich needs to addressed first.
most likely r134-a if it has screw on connections it would be r-12