Rather than calling in the plumbers to replace a faulty faucet, you can easily perform this plumbing task yourself as long as you have a pipe wrench, pliers and a Philips head screwdriver; three tools that are commonly found in most homes.
The very first thing to do is to locate and turn off the water to the faucet. The valve for this is usually found directly under the sink. Rotate it till it is firmly shut, then open the faucet to make sure that there is no water coming out.
Next unscrew or pop off the cap on the handle so you can get to the screw that is hidden underneath. Once the cap is off, unscrew the screw that attaches the handle to the valve. When you have taken it out you should be able to take the handle off. You will then see another cover which is over your valve stem assembly parts. Unscrew that cover and take it off.
You will then be able to remove your valve stem and the assembly. First unscrew the nut on top of it with your wrench, and then unscrew the whole vale stem assembly part with pliers or a wrench. Once it is removed you should see a washer, also held in place with a screw. Undo this screw and then take the washer out. You will then be able to put the new washer in place.
Finally reverse all the steps above until the unit is assembled again. Once you are sure everything is tightened you can turn on the water and open your faucet to make sure that it is working properly. You may also want to replace any screws or nuts that have deteriorated while replacing the washer. If you are unsure what size washer your faucet uses you can leave the faucet un-assembled and take the washer with you to a home repair shop. That way you can ensure you get the correctly sized washer.