1990 Dodge Dakota What to do about repaiering my air condit

I recently had a mechanic try to put some freon in my air conditioner in my truck,and when he did the pop off valve behind the compressor went off. It made a terrible popping noise.One guy told me to take it to an air condition mechanic to have them drain the line and put some more freon in it.I am so paranoid and to old to do it myself so I'm throwing myaself at your mercy.Please help me.(Old , burning up & frustrated.)