Depends on R12 or R134a...

There are two types of freon - R12 and neweer R134a. First you might need to find out where is leaking - before a full charge of freon, charge about 1 lb of the same type freon with dye and let it circulate. Use a leaking detector (uv light) to find out where the leaks taking place. Fix it - some times you need to change compressor, drier and expension valve. Never mix R12 with R134a.

After that, you might need to clean/evaculate the oil/residue/freon, (If your car is filled with R12 (older cars most likely contain that), then refill R134a (R12 is extremely expensive now). You also need to buy the adapters before conversion.

I changed compressor on 300SDL.