R134 is used, but have the leak fixed first.
It's the schrader valve on the low pressure side of the system. Find the AC compressor. There are two hoses coming out of it - one is larger than the other. That's the low pressure hose. Trace it back until you find the scrader valve - it's usually topped with a plastic cap.
Buy a kit at Wal-Mart or auto parts comes with instructions While the equipment to install refrigerant is widely available, I try to discourage the practice for a few reasons: System charges are very specific...too much and your compressor will run too hot. Too little, and the lubrication to the compressor will be affected. Either means a short life span for the compressor, and a future expensive repair. Refrigerant is a controlled gas, and it is illegal to vent it to the atmosphere. Refrigerant acheives very low temperature at atmospheric pressure and can hurt you. If there is a mechanical problem, adding refrigerant won't help. If there is a leak, adding refrigerant won't last long. Please, have a professional help you.