It’s not uncommon for a brand new shut-off valve to leak around the handle area whenever you turn it on. is probably occurring around the valve stem. The valve stem passes through a “packing” nut and washer, which provide a watertight seal. In the “old” days, fiber was packed tightly around the stem, hence the name.
First try tightening the packing nut one-eighth to one-quarter turn with a wrench (Photo 1). On a new valve, this almost always stops the leak. If it doesn’t, the packing washer is probably damaged and will need replacement.
Here’s how. First shut off the water to the leaking valve. Next remove the handle from the stem and then loosen and remove the packing nut (Photo 2). Remove the old washer and slide on the new one. Reinstall the packing nut, tighten lightly with a wrench (don’t over tighten at this or any other point) and reattach the handle. Turn the water back on, open the repaired valve and check for leaks. If it’s leaking, tighten the packing nut one-eighth of a turn at a time until the leak stops.