How do I fix the problem "water hammer"

Depends, what kind of pipe? Where is the noise? Is it a loud thump or more like a ratatatat ? Does it do it with the toilet stops turned off?

Water hammer happens when you turn OFF a water flow suddenly!!

I think the best way to explain this is at a washing machine there are 2 valves

The hot and cold water valves should have a tee that has a valve out from wall

There should be a pipe that extends up above the tee, about 1 foot and is capped

When washer needs water the valve in washer opens and fills machine

When the valve shuts off water tries to rush into hammer

But it meets an air pocket that silences the noise

Water Hammer & Plumbing Pipe Noises (http://www.factsfacts.com/MyHomeRepair/PipeNoises.htm)