Assuming that inside shutoff is a multi-turn valve, the washer or the valve seat are likely shot. Sometimes, all it will take is to replace the stem washer, but if the seat is shot, it may not be replaceable, and you may need to replace the whole shutoff. Preferably, you'd use a 1/4-turn ball valve, as they tend to last much longer than a valve with a washer.
To replace the washer, you'd have to turn the house water supply off, drain off the water in the pipes, then open that faucet part way, unscrew the packing nut (the nut the handle goes through), then when loose, back the handle and stem out of the valve body. The washer will be on the end of that stem (shaft) of the valve. Most are held in with a screw through them in the middle. The screw might be messed up and also need to be replaced. The parts are cheap, but while you have it out, look into the valve and see where that washer goes - that is the seat, and it should be smooth. If it is rough, the new washer will get eaten up and not seal.