Do you know how to make events with conditions and add actions to them?
It's pretty easy if you understand how to work with C2..
Add keyboard to your project
on key pressed (choose which key) play punching animation
(first you would have to have created the animation ofcourse and import it into C2 with the image editor)
Add a collision box to your animation (either by the pin behaviour or by setting the box to an imagepoint on the frames the punch is punching)
if collisionbox is overlapping enemy - enemy subtract 1 from variable health..
But if this is all too complicated to understand I would suggest following the beginners tutorials to learn how to program with construct2..