You need to replace the valve where the knobs are attached. Ask them at home depot
My Bathtub Faucet Won't Stop Leaking. How Can I Fix It?
The easiest way is to turn off the water to the house and open some faucets to drop the water pressure. Remove the handle then the cartridge which is held in place by either a retaining clip or a large nut which can be unscrewed. You may need to temporarily reattach the handle in order to pull out the cartridge. Bring the cartridge to a hardware store to find a replacement.
Replace the washer in the faucet - you should be able to purchase washers at your general store or hardware store - there are packets with different size washers available, then be sure to have the water turned off at the 'toby' or outside the property - you may need to ask the landlord or someone where that is, remove the tap fittings, you'll see the worn out washer on the tap so just replace it with a new one and put everything back together again..
