The first thing you need to know is what kind of refrigerant is in the system, since there are a few different ones. If it's truly freon (R-12), good luck trying to find it for less than the cost of a new fridge.
The second question you need to answer is whether it's low on refrigerant, or is the compressor worn out and not providing an adequate pressure differential.
The second question you need to answer is whether it's low on refrigerant, or is the compressor worn out and not providing an adequate pressure differential.