what is the best way to find a reliable plumber to fix a heater?I had a co-worker's father who was a plumber

come over,and all he did was make it worse,He told me certain things need to be done to it,they were,I find out later from other people,that what he said that needed to be fixed didn't.I won't go back to my co-worker's dad who is a plumber,that is for sure,any suggestions? The heater is a gas,wall heater.