Head injuries may happen because of fall, accidents, while playing sports or physical assault. It may be serious or minor.
Minor one can be attended at home but serious injuries require immediate hospitalisation.
The crucial part is how to provide a person with serious head injury first aid or an aid which a person requires before reaching hospital.
First for all practical reason treat the injury as a spinal one and keep the body and head straight, if the person is unconscious check his pulse, if necessary provide cpr, tie the wounded portion with clean piece of cloth. If it gets filled with blood, don't remove it. Tie another one. If possible apply ice to swollen area.
Remain in touch with emergency rescue team stationed at the hospital.