Almost all pedals all use a 15 mm open end wrench. If the flats are wide enough, a regular mechanics wrench will do. If not, there are specific pedal wrenches with narrow jaws to fit them. Park makes a very good one but there are others. A few pedals have a 6 mm hex recess in the inside face of the spindle but that's not common.
Remember the right pedal (drive side) has regular right-hand threads (counterclockwise to remove) while the left pedal (non-driveside) has left-hand threads (clockwise to remove). One way to keep them straight is to position the wrench handle pointing straight up. Then both pedals unscrew by turning the wrench toward the rear of the bike.