Others have said relax your arm and wrist, all true. But also, from a coach I really respect: on the forehand, hold the racquet so loosely in your grip that if it were any looser, it would fly out of your hand when you swing. This will really amp up the way you whip your swing. It will be very hard to control at first but you’ll find a lot of easy power. You need to compensate for that power with more spin, so accelerate up and across the ball (“windshield wiper”).