I don't know your level and I am not an expert. Punching looks a lot easier than it really is, take quite a few month to get into it, not just punch. It is better to have someone that do boxing to guide you.
In short, you have to learn to use your whole body, from legs, hip, waist, shoulder to generate power. The arms are more like a whip and only tense up at the point of contact.
When you get your body to work with the punch, you first start out just barely tap the bag and pull back. You want to pull back as fast as you punch. When you get the feel, then try punching like a little deeper(say 1/2") and pull back. When you get good, then go a little deeper. Don't try to punch hard, just lightly tap the bag.
It's a few months thing, do not rush. Get someone into boxing to guide, I can only tell you how I do it.
