Exam and history. Too vague a question without having a thorough history and an examination. Too many variables when attempting to determine the cause of a headache. Have you seen a doctor. You might also keep a pain diary and note how you feel throughout the day.
Need more info. What precipitates ha, what do they feel like (sharp, dull, etc.), other symptoms? Are you upset about anything (social relations, job, other)? These may be tension ha, usually beginning in back of head and radiating forward along the sides to forehead. Relaxation therapy may help, but if this persists and is quite severe, you should be seen by a doctor.