We need to know as much as possible to help work out what the problem is. Most fish illness is caused by poor water quality which is why we always check that.
How big is the tank?
Does it have a filter? If it does, is the water flow gentle or strong?
Does it have a heater?
What are the levels of ammonia, nitrite and nitrate in the water? If you don't have testers, take some tank water to a fish store and ask them to test - but make them give you the numbers rather than something like 'fine' or 'a bit high'
Did you cycle the tank before getting the fish? By this I mean did you grow the good bacteria by adding ammonia to the tank before you got him?
How often to you do a water change, and how much do you change?
One of the most simple 'treatments' is lots of large water changes. This is the first thing to try when a fish is not well.