Question

How hard to i need to hit my head to die?

Answer 1

You would need to hit it hard enough to cause your brain to bleed for best results you should fracture your skull 

Answer 2

To die from impact to the head you must strike the head hard enough to cause the brain to hit the skull hard enough. Not causing a concussion but rather a severe trauma to the brain causing massive brain cell loss. Another sweet spot of the head is the temples on the side of your head. A hard enough hit to those will surely cause permanent damage or even death.