You should NOT attempt to add refrigerant yourself. Adding refrigerant requires specialized equipment to measure the system's pressure and vacuum it out before adding the correct amount. Improperly charging the system can damage the compressor and other components, leading to costly repairs.
To recharge your Chevy Tracker's AC system, you should take it to a qualified mechanic or an automotive AC specialist. They have the tools and expertise to diagnose the problem (it's not always just low refrigerant), properly evacuate and recharge the system with the correct type and amount of refrigerant.