Firstly if it's still an R12 system, you need to retrofit to R134. You need to know exactly how much refrigerant the system holds (in weight) which should be on a sticker in the engine compartment. You also need to know how much if any refrigerant is already in the system.
Honestly you're probably better off letting a shop equipped for A/C work check it out. It's not cheap but neither is blindly shooting a can of freon into the system when you don't know how much is already in there, if there's a leak, or if a low charge is even the problem. And venting off freon into the air is a no-no. There are plenty of other possible issues. Could be a problem with the low/high pressure cutoff switch, expansion valve, compressor clutch, etc.