My hot water heater piolet light is not lighting what can we do

I will assume you have tried to relight it? Does it never light or go out after lighting? If you tried and it goes out when you let up the knob, you probably need a new thermocouple. They are attached to the gas valve and sit in the pilot flame. They are less than $10 and are prettty much universal. Instructions come with the part. If that fails, you may need a new gas valve although 95+% of the time it's the thermocouple.