I recommend watching some fighting videos or animations…

If you don’t want to do that then I’ll try my best…

First off, you have to make it look like they charge up. If they just lazily throw their hand at the person, it looks bad. Try throwing the figures’ hand back (not too far) or simply bending their arm back, tucking the forearm into the chest. I don’t really like this way, but it’s the only option for higher quality figures. When using a simple figure, you can stretch the punching arm back and the front arm forward, so it almost looks like they are blocking, but preparing the other fist for a punch.
In the second motion of the movement, bring the front arm back and push the body forward. It seems as they have lunged their full weight into their fist. Give this a couple of tries until the motion looks fluid and realistic.
The punch is quite simple, bringing the fist forward to hit the face. For more tips on the punch, move the camera (slightly) to make it look like they got hit really hard. Bring the head back and let the body follow, as though the body is a rope being forced into going wherever the head goes.
The hand follows through with the punch and gradually slows to a stop, until it falls by the figure’s side.
If you are still confused then watch some YouTube. They got some good shit over ther