In attempting to replace the water line for the hot water to the kitchen faucet, I broke the stem off the valve. My problem is that I can't identify how to shut off the hot water flow to this point so that I could disassemble the valve in order to replace the stem. Where can I look for a valve or a shut off ahead of this valve?

You can do it two ways if this is the hot side, close the inlet S/O valve to water heater open the hot side of the faucet until all water is drained, then work on the angle valve or close the main water shut off valve to the house, to replace the parts on the angle valve see the image below, Thanks.

John