The first thing your going to need to do is turn off the water to the faucet. There should be 2 shut off valves under the sink. One is for the hot and one is for the cold. Reach under the sink and turn them both off.Turn them clockwise until they stop. These valves are sometimes hard to turn due to the calcium build up on the valve stem. I you are unable to turn the valves under the sink off. You will need to shut the water off to the entire house.
If you have an elecric water heater you must turn the electricity off to it before turning the water off. Usually there is a circuit breaker labeled "Water Heater". Just switch it off.
To locate the main shut off to your house. Walk out to the water meter and walk or look straight toward the house.There should be a hose bib (a place to hook up a hose)with a gate valve below it in line with the meter. Turn that valve clockwise until it stops. Then open the hose bib to let the pressure off the line. Your water is now off. Leave it off until you have either repaired the leak or your plumber gets there.Turn the hose bib off after you have turned the water back on.