Under your hood there will be some silver tubing probably close to the windshield. In line with the tubing you will see a service valve. Its a small cylinder with a plastic screw on cover, maybe even a tag. Unscrew that cover and you can add refrigerant there. You can buy it at any auto parts store, most likely it will be R-134. Buy the bottle with the gauge built in to it and follow the instructions. Be careful not to over charge the system, the can will tell you the proper PSI. Also double check with your owners manual or the labels under the hood and it should give you a diagram of the A/C system and tell you the proper refrigerant to use.