But you dont need to call a plumber and spend money on a service call with a few easy steps you can become your own master plumber
The first thing you need to do is turn off the water supply to the sink, most of the time the valve is located right underneath, if not you need to follow the plumbing line to the nearest shut off valve. If the valve resists being turned, put a drop or two of oil right where the stem disappears into the large nut. Then use a wrench to loosen the nut one complete turn, and tighten it back up by hand. Wait for a couple of minutes and then you should be able to turn the valve off.
The second thing to do is remove the stem from the top of the fixture, then there will be nut that needs to be turned counterclockwise to be taken off. Grasp the handle and turn it counterclockwise, as if you were turning on the water. The whole thing will lift out in your hand. If you have a ball-type fixture, there will be a setscrew on the lower surface of the handle itself, remove the screw. There will be a plastic ring or cap just unscrew and remove then pull out the ball assembly.
Your almost finished, just need to replace the washer, which should...