You need to replace the washers on the ends of the faucet stems, and possibly the seats that the washers seal against.
You will probably need a set of faucet socket valves to find the one that fits your valves (these can be found at most any hardware or big box plumvbing section...they're not expensive).
You also may need an inexpensive seat tool to remove the seats if they're pitted and corroded.
Turn the water OFF, remove both valves (and the seats if necessary), take them to a store and get washers (and seats) that match. The valves also may need new o-rings.