A number of things can result in a topped shot, but a primary concern is balance. If you focus on nailing the finishing position in perfect balance, with the weight almost entirely on the target side foot at the end with the body facing the target, this should allow the club to impact the ball on a descending arc. This, and keeping your center of gravity “quiet” during the swing, should go a long way to correcting the problem.
A good drill is to swing the club with the eyes closed, brushing the grass and finishing solid. This will give a greater sense of any movement your center of gravity may have. During this drill, you have only two goals–nothing else matters. 1) brush the grass, and 2) “stick” the finish, like a gymnast dismounting from the high bar to nail a perfect “10”.