The refrigerant that was originally in your refrigerator is freon 12. This was deemed to be the most harmful type of refrigerant for the atmosphere. There is a large fine for releasing it into the atmosphere, and you need a license to buy and use it.
If the freon has leaked out it wouldn't do any good to add more, as it will leak out again. The refrigerant system for the refrigerator is hermetically sealed and should never leak. You would need a licensed technician to find and repair the leak and recharge the system. This would be fairly expensive, and your best investment would probably be to replace the refrigerator.