The pilot reported that he was performing a soft-field takeoff in preparation for his upcoming private pilot checkride when after the nose wheel lifted off the runway, the airplane yawed to the left. He applied right rudder in an attempt to maintain control; however, when a runway excursion seemed imminent, he applied full brake pressure on both pedals. The airplane veered “sharply” to the left, exited the runway, and impacted a ditch before coming to rest in an adjacent field. The airplane sustained substantial damage to the forward fuselage and left wing. The pilot reported no preaccident mechanical failures or malfunctions that would have precluded normal operation. The pilot reported that attempting to steer the airplane back toward the runway centerline and not applying the brakes might have prevented the accident.