The single handle sits on a ball assembly. When the faucet is raised, this ball rotates to control the flow of water. Moving the handle to the left or right allows hot or cold water into the mixing chamber. Leaks at the handle may mean that the adjusting ring is loose or that the seal above the ball is worn. Leaks under the spout may require replacing the o-rings.
Repair kits are available at plumbing supply stores.
Unscrew the cap, using cloth or tape- covered adjustable pliers if needed. If you have a swivel-spout faucet, lift off the spout as well. You may have to carefully pry the spout loose at the base or exert some pressure to lift it off, since it is friction-fitted around the body. If the faucet has a spray diverter, replace the diverter o-ring in the same way. The key on the cam assembly also fits into a matching notch on the faucet body.