Shut off the main
If you live in a house, the water meter is near the front boundary. Shut the water off here, then turn on a tap inside to check the water has stopped. If it hasn’t, call a plumber.
Dismantle the tap
With single taps, remove the handle, unscrew the body cover and use a shifter to remove the body and spindle.
Replace the washers
Replace the body washer, O-ring on the spindle and the washer that should drop out of the spindle (use needle-nose pliers). Lubricate spindle with petroleum jelly and reassemble.