[f5fstop]

I suggest taking it to a service location that can service the AC; it does not have to be a dealer.
the system uses R134a refrigerant, but it just does not wear out, if it is low, it has leaked out. Adding more without fixing the leak will do you no good. THAT IS, if there is a leak.
There are other reasons it may not be operating such as a high/low side pressure switch broken, a compressor clutch that won't engage the compressor.
That's why, I would suggest you have someone who knows AC systems, look at the system. It may cost, but it will be less expensive in the long run, than throwing money at items that do not fix the problem.