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

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

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.