It is a sometimes overlooked thing these days. The proper boxing fist is not only designed to inflict injury to your opponent but also to PREVENT injury to yourself. There are several things to remember when preparing to hit someone.

First: Violence is never the answer. It is a last resort and therefore is not condoned.
Second: If you do make a fist, be prepared to use it. It IS a weapon. Just like a pistol. Do not brandish it unless you intend to use it.

Here is the proper fist and arm for delivering a solid punch. 

Step one. Make a fist. Not the silly thumb inside the hand flex like a pro bodybuilder fist. The kind in the first picture. See how my thumb is tucked over my index finger and not in the way? That is how yours should be. If it is inside your fist it will break when you hit something. NOT GOOD FOR YOU!

Step two. The ulna (wrist bone on the little finger side of the forearm) should be aligned with the outside edge of your fist. This prevents spraining or  straining the ligaments in the wrist upon contact with the enemy face.

Step three. The top of the fist (back of the hand) should be aligned with the top of the forearm. This prevents wrist fractures when something gets hit. 

Just remember this. The best way to avoid a fight is to take your pants off. NOBODY wants to fight the guy with no pants.