There has been many topics about this so you may wanna use the serch feature.
Basicly your punch power is made up of
speed
mass
skill
body conditioning
posture
your skill improves with many diffrent kinds of practice.
speed improves with skill and muscular strength
mass is the weight behind the punch, i.e. how much you weigh.
body conditioning is improved buy punching solid objects, its how your body deals with a stopping force, i.e. your joints. This is very closely linked to posture.
your posture controls how much resistance you give against your target.
so to improve you punch you have to practice for skill, get stronger and put on weight and have better posture.
for weight training, its very bad for you to train one area of your body more than others, so if your work on pressing, your chest will tighten and your shoulders will roll forward, so your back will be arched. This is why you must train all muscles the same.