I have a headache. what can be the possible causes for it?

Need more info: Where, when, how bad, other symptoms...?

See your doctor: Headaches can be caused by a number of possibilities. These include hormonal fluctuations, dehydration, muscle tension, sinus/allergy issues, infections, migraines, tumors, and a number of other medical issues. Best to see your doctor to narrow the cause.

Headaches: Your headaches could be coming from tension, dehydration, lack of sleep/poor sleep, skipping meals, stress, teeth grindgin, sleep apnea/snoring, or chronic sinus problems.

Get checked out: A good start at finding potential headache triggers would be to maintain a diary. Every time you have a headache write down what you were doing, the foods you ate that day, alcohol, smoking, caffeine, stress, how much sleep you had the night before, eye strain, etc. Try to find patterns in your headache so that you can learn what things to avoid. Get checked by your doctor.