First you need to determine if the freon in your car is R134A or R12. In order to replace the freon in a car that has R12 (and yes there are still very many cars out there that still have R12!) you would require a license and certification, the proper recycling equipment and disposal equipment of old freon. To do any work on an R12 system without these things can lead to big fines (sometimes as much and more than $15,000 per use incident), that is if you're caught!On an R134A system you should ad freon only after you have determined #1. The extent of the systems leak. No point putting in freon just to have it leak out 5 minutes later. #2. The integrity and overall condition of all hoses and belts, and any components that are visible. #3. If the compressor and it's clutch are still turning and not frozen up. After doing these initial checks then a set of gauges for R134A would be most beneficial. Although you can purchase cans of freon with the hose/gauge/valve in on piece. A bifold set of gauges is an excellent diagnostic tool and can help prevent spending money on something you may not need, providing of coarse you understand the applicable readings.