If it is the sink, there is usually a shutoff valve under the sink (in the cabinet). If it's the tub, that may be more difficult and might require shutting off the water at the water heater.
You can usually pick up packets of washers that have several different sizes in them for a couple of bucks at the hardware store. It usually only takes a screw driver and a pair of pliers to replace the washer.
Simply a Rose to brighten your day,
And maybe lessen the cares in your way;
And also, too, to help you to know,
That in knowing you, many others grow!
Change your washer in it.