If you need to make a plumbing repair to your bathroom sink, you will need to turn off the water to that sink prior to initiating the repair. Bathroom sinks have shut-off valves located under the sink. You turn off the water to the sink by closing the shut-off valve to the sink.
Find the shutoff valves located on the pipe below your bathroom sink. If you have a cabinet under your sink, look inside the cabinet to find the shut-off valves. Most sinks have two shut-off valves, one for hot water and one for cold water.
Close the shut-off valves by rotating the knob in a clockwise position. Use your hand to tighten the shut-off valve. You don't want to over-tighten or you could damage the shut-off valve.
Turn on the hot and cold water on the faucet attached to your bathroom sink. Allow any water that was stuck in the pipe between the faucet and the shut-off valve to drain out.