To hit a tennis ball with topspin, at the point of contact with the racquet being perpendicular to the ground, twist your wrist so the face of the racquet with the point of contact with the ball...
On a personal level, it all depends on your skill. As a comparison, the fastest a professional has hit a forehand was 120 mph (by Gael Monfils), and 155 mph on a serve (by Andy Roddick).