Your car should be refrigerant leakage problem. You can't only re-charge the refrigerant to the system. You must find out the leakage first.
But if you don't know the operation of air-conditioning system. -Don't charge the refrigerant yourself, because it may cause explosion of the refrigerant cylinder.
I suggest you go to some big maintenance centre.
right procedure of maintenance centre is :1. find leakage & rectify. 2. perform vaccum of system. 3. re-charge refrigerant.