Turn off the tap’s water supply or the main stop tap before removing any parts, and place a plug in the basin to catch screws. For a traditional tap, replacing a worn washer or O-ring may stop a drip; mixer taps often need a replacement cartridge instead. Clean away mineral build-up and check that connections are not cross-threaded or overtightened. If the leak continues, water is coming from inside the wall, or you are unsure which part to replace, contact a licensed plumber for advice.