Normally you'd use the can stuff (R-134A A/C Refrigerant) to recharge your system. The problem is, if your system is empty, then there must be a leak. If you just charge it up again, odds are your refrigerant will just leak back out again.
You can buy R-134A at places like Autozone or Pep Boys and they can help you put it into your car, or at least watch you as you do it. (If they aren't too busy) They also sell R-134A with additives to stop very small leaks, so you can try that.
However, if you aren't already familiar with A/C systems in modern cars, I recommend you bring it to a shop, A/C is one of the most finicky systems on a car when it doesn't work. There are expansion valves/orifice tube that can get blocked, etc and trying to solve the problem yourself can be more trouble than its worth sometimes.
If you have a plug in your system, then adding more R-134A to your A/C isn't going to help at all, and you may damage the system by overcharging it, to the point where it literally explodes (thankfully not with fire) in your face.
