The basic thing is to find a leak. That usually involves using dyed fluid and an UV light. And of course inspection of the compressor to make sure it's working.
One way to see if you have a leak is to turn the a/c on, if the high speed fan does not turn on, then you have no/little pressure in the system, meaning you have a leak.
Basically, just find a reputable shop to take it too. Expect the cost to be between $500 and $1000, as usually more than one thing will be replaced to ensure proper operation of the system in the future (this is standard practice and not a rip-off). Working on an a/c system in your backyard without having a lot of know how and proper equipment is not a good idea.
