Locating the suction and compression side ports when performing an air conditioner recharge is the first thing you need to do. These will be located in the engine compartment. The suction side will be before the compressor, usually on the driver side of the vehicle. While the pressure side will be after the compressor, normally on or near the receiver/dryer, which is a silver colored device. The low, or suction side fitting will be the larger diameter fitting. Usually this will be a ¼ diameter flare type fitting. Obviously, the high side is the smaller fitting. This is usually a 3/16 inch flare fitting.
For more info, see How to Recharge the Air Conditioner in Your Car