Go to an automotive a/c shop. As said earlier, there is a lot of "junk" put in these systems, so if you have not had the car for a long time, the shop can connect their refrigerant identifier instrument and tell you for sure if it is not contaminated.
They can then check for leaks and charge the system. As wisely said earlier in the thread, if you are not knowledgable regarding refrigeration and do not have the equipment, this is a job to put in the hands of the automotive a/c shop.
If the shop you go to suggests alternative refrigerants such as "Freeze 12, Duracool, Autofrost, etc," RUN, don't walk away from that shop and find a shop that uses ONLY R12 or R134a.