We have a waterpik handheld showerhead that is dripping. I shut off the water at the street, and the inlet valve on our instant hot water heater. The water at the faucets is off. I turned the shower faucet on to make sure water wouldn't flow, but water is flowing slowly out of the showerhead. Even when I turn the handle off and on again, it makes the gushing noise and more water comes out and then it slows down but never stops. How do I stop the water from coming out of the showerhead?
It sounds like you have more than 1 bad valve. The valve to the home is leaking and the ones to the shower are leaking. If really old you might turn a bit harder for a gate valve, if that won’t shut it off open a hose bib they are usually the lowest point on the system. Then pull the faucet stems and replace the washers. After replacing the washers on the shower I would be checking the shut off to the water heater also many have a cold water main shutoff at that location change that washer also. If you can shut off at the street then change the washer at the house shutoff and you should have fixed everything.
I purchased a kit with 2 or 3 different sized washers they look like little volcanos These are healed in place with brass screws my kit came with some of those also.
Over the years I have repaired hundreds of valves mostly with the washers and some valve stem packing I replace them and still have the metal box (tells you how old it is)
Every home owner should get a kit like this only 10-20$ and start adding as you do other valves, mine also has orings for washerless faucets now you can fix the valves in just a couple minutes in most cases for under a dollar.
Worst case you may need a seat cutter / reamer but most of the time a new washer will do the job.
I am sure there are videos online to provide live examples on how to do it.