To fix a leaky faucet in your bathroom, you will need to gather a few tools and supplies, including a wrench or pliers, a replacement washer or O-ring, and some plumber's grease. Then follow these steps:
Shut off the water supply to the faucet by turning the valves under the sink clockwise.
Use the wrench or pliers to remove the handle of the faucet. You may need to remove a screw or other fastener to do this.
Once the handle is off, you will be able to see the inner workings of the faucet. Locate the washer or O-ring that is causing the leak, and remove it.
Take the replacement washer or O-ring with you to the hardware store to ensure that you get the right size and type.
Once you have the replacement washer or O-ring, coat it with a thin layer of plumber's grease to help it seal properly.
Install the replacement washer or O-ring in the faucet, making sure it is seated properly.
Reassemble the faucet, using the wrench or pliers to tighten any bolts or screws.
Turn the water supply back on and test the faucet to make sure it is no longer leaking.
If the leak persists, or if you are unable to locate the source of the leak, it may be best to call a plumber for assistance.