You need to buy (or borrow) a water valve key. They look like a capital "T", with a double steel prong on the bottom that tightly slides over the valve's fin. The flat portion is grasped, one hand on each leg, and turned. Usually just far enough so the fin is perpendicular to the pipe, or 90 degrees. If the valve hasn't been used in a long time, it may have to be freed up by giving it a few stout taps (from the top, straight down) with a hammer first, before trying to turn it.