You don’t have to worry. Fixing it is quite easy. Firstly tighten the packing nut slightly with a wrench. Then test the valve. If it still leaks tighten the valve and then try again. It is actually quite common for brand new shut-off valves to leak whenever you try to turn it on. This probably occurs around the valve stem. The valve stem passes through a nut and washer which provides a water-tight seal. Try tightening the packing nut one-eighth to one-quarter turn with a wrench. If the leak still does not stop the packing washer is probably damaged and will need replacement. First shut off the water to the leaking valve. Next remove the handle from the stem and then loosen and remove the packing nut.Remove the old washer and slide on the new one. Reinstall the packing nut, tighten lightly with a wrench and reattach the handle. Turn the water back on, open the repaired valve and check for leaks. If it is still leaking tighten the packing nut once again one-eighth of a turn until the leak stops. If that does not help you could try calling for professional help.