Sound like your low on R-134. That is what they use for the refrigerant these days. You can buy a can at the parts store for $30 to $50 range. You will also need the hose attachment to put it into the system. The directions are fairly easy to follow. there is 2 ports in the system. The attachment will only fit one which is the low side. It has a blue cap most of the time. if you dont want to try it yourself then you will have to go to a shop. one other thing remains. Why did the refrigerant escape? you will lose a bit over time. If you do it yourself and it is only fixed for a short time then you might just have a leak. You will want to go to a shop then.