The football helmet is only protective gear. It is not an offensive weapon. It's only purpose should be to protect the player's head and face. The player should never initiate the contact with the opposing player with his football helmet.
Correct fit - A football helmet is only as good as the fit on the player. It should not be too tight, that is it should not be uncomfortable or squeeze the head at any point. (It should not leave any visible wearing marks from just wearing the helmet) On the other hand it should not be too loose: It should not move when the player is running. And the football helmet should be still in place after 'normal contact' so that the player is not looking out of the earhole after almost any contact.
Safety
The evaluation of any athlete, whether as a part of health evaluations prior to activity or as a diagnosis of an injury as the consequence of sports activities, is specific to that individual and the history and current state of the individual presented.
Advice, diagnosis and treatment is individualized according to numerous factors, including patient health and age information, medical history and symptoms. All athletes should be cleared by a physician or other appropriate medical professional before engaging in physical activities and, after injury, diagnosis and treatment, for return to play.