If your ac in your car isn't cooling and your refrigerant leaked out then just putting new refrigerant into your ac system won't fix the problem. It will just leak back out again.
You need to find the leak and get it repaired first. You should take your car to an ac repair place and have them look the ac system over on your car so they can find and fix the leak and then have them put refrigerant into your system.
Ac systems on cars can be under high pressure and should only be serviced by qualified persons.