You've given us very little information to work with, so this may or may not help. If this is a frost free faucet, one of the very common problems is the result of leaving a hose attached over the winter. The hose prevents water from draining from the faucet tube that runs through the wall and attaches to the supply line. Since the actual shut-off parts are at the inside end of this tube, the leak doesn't show up until the faucet is turned on. If this is what your problem is, the leak is probably much heavier inside the house or wall than you are seeing on the outside. If this is your problem, the only repair is to replace. This must be done from the inside where the faucet attaches to the supply line. Sometimes these are soldered, often they are threaded. If threaded, use two wrenches to avoid twisting the supply line. Lots of "ifs", but without more data, pretty hard to do more that a generic guess.