Your best bet would be to find a reputable contractor to come and perform a diagnostic. There is normally a charge to get the technician to your home (diagnostic) and then additional charges may be incurred depending on the repairs that may be needed. Some contractors charge a flat rate (recommended) and some charge and hourly rate. If it does indeed need a freon charge, they usually charge by the pound (3lb. freon, etc.).
If you can get a referral from a friend, great. If not, definitely call a couple of contractors before making a choice. Besides the internet, you can always rely on the good old fashioned yellow pages. Good luck!