What should you do if a person is unconscious and not breathing?

Call 911! Place the person on their back, perform chest compressions, firmly push downwards in the middle of their chest and then release

There are several action you should do:

Call 911. Check his vital signs. Look for physical injury. Put the person in a comfortable position if there's no physical injuries. Do Cardiopulmonary Resuscitation.

These are the basics you should do, also checking is breathing is included, but in this case, the person is not breathing already.