Being a Mechanical Engineer, I can understand how Air conditioner works. There is no need to charge gas or refrigerant (which can be R-22,R-12 or R-134-a (Latest)) if your AC works well or even at lower performance level. There are many components in AC which might degrade the performance such as compressor, condenser and capillary tube. You should first consult technician of refrigeration and air conditioner. So, do not simply blame gas when your AC's performance is down.
Second thing, never try to fill or charge gas/refrigerant on your own. It is very RISKY. Some refrigerants are very toxic. It may harm to your health too. And also you cannot always get 100% pure refrigerant/gas once your AC is depleted with gas.
Now coming to the procedure, You should do the followings to charge the gas in AC.
1. First find out the outlet with valve where we can exhaust the gas from AC.
2. You need to evacuate the system by vacuum pump. This involves removal of moisture and air entrapped during exhaust process.
3. Get mass flow meter ready and connect it in line between compressor and charging cylinder. Start the charging process initially slowly and then at required rate. You cannot fill more than recommended quantity as pressure-volume relation might change the phase change process for refrigerant.
This is what you should know.