I will advice you 2 things.
1. A proper front or horizontal punch strikes with the 1st 2 knuckles (fore and middle finger) with the writst aligned with the forearm, no bending.
2. If you want to maximize your punching power, angle your fist to a 45 degree angle. That means, do not turn your fist al the way until is horizontal; strike with the punch between a vertical and horizontal position; the 45 degree punch.
Try these and see how you get better at punching!
Yours in Kenpo,
Carlos M. Roman, H.I. American Kenpo Karate Studios