Some times stress, Hormonal imbalance, malnutrition, oxidative stress, life style, these are the reasons behind the getting skin darker.
Identify any relative issue in your case. Take healthy food, more food which have more antioxidant. Use sunscreen lotion with spf50 before going out side. Use face wash which contain glycolic acid. Some doctors advice glutathione Tab. U can try it after consulting any doctor.
Health Tips
Avoid junk food, oily,spicy items. Wash your face two times in a day with face wash. Avoid direct contact with sunlight.