You can purchase small cans of R134a and some of them will come, or you can purchase seperately, a charging hose. On the A/C lines towards the passenger side of the engine bay, there will be two service ports. Typically, the low side, which is the one you want to use, is the thicker of the two lines. Also note, that the fitting on the charge hose will only fit on the low side service port, so if it won't fit, try the other port.
Start the car and turn the A/C on. Once the compressor starts running, it will start to pull the refrigerant out of the can.