When a friend removed the sink in my powder room he twisted the shut off valve to the point that I need to replace it. I need to know how to turn off the hot water at the hotwater heater so that I can cut the copper pipe and replace the shut off valve
I have turned off the main valve to the house but is this enough to be able to stop the hot water flow to the bathroom sink?

I need to know how to turn off the hot water at the hotwater heater. There should be a shut off valve located on the cold water supply going in to the heater.
What type of angle stop do you have? Compression, sweat or threaded? If it's a compression type.(see image) The repair's a easy one, But I have to know. Back to you. Tom

Novice here! The valve and pipe going into the top of the heater tank is the round kind. Like you would see on shutoff valve for water to the washer or outside faucet. The only valve looking like your sample is the gas shut off

How to shut off the water heater.
If your shut off to the house is 100% you should be able to remove the old angle stop shut off valve and install a new one without a lot of water draining out. Open up the hosebibbs or the lowest faucet to the ground to help drain residual water in the lines. One exception would be if the faucet you are working on is located right next to the water heater.
You can find out more about angle stops here. Angle stops (http://www.plumbinglessons.com/sup/anglestop.htm)

Novice here! The valve and pipe going into the top of the heater tank is the round kind. Like you would see on shutoff valve for water to the washer or outside faucet. The only valve looking like your sample is the gas shut off

Pat the stop I put up is the angle stop in your powder room. This is the one,(see iimage) on the coold water inlet to your water heater. Either turn that o9ne off or you could get the same results by shutting down the entire house. Good luck, Tom

To be on the safe side If you have a main shut off valve to your house close that one it will shut off the water to house, Open up the hosebibbs or the lowest faucet to the ground to help drain residual water in the lines, see the image below will show how to remove the angle valve, good luck.

John