Why is my skin so dark from my forehead, what shall I do to lightn it?

Lighten dark pigment. What is your skin color? This is important when thinking about procedures that will help balance your pigmentation. As a general rule, no matter your skin color, the daily use of an antioxidant sunscreen is paramount. If you work outside, or have a lot of outdoor activities, wear a wide brim hat in addition to your sunscreen. Peels, ipl, laser peels, skin lighteners are options for pigmentation.
Inflammtaion,ethnic. This could be variation of ethnic skin or more probably secondary to inflammation. Treatments would include antiinflammatory combined with peeling agent such as retionid, vitamin a derivative. Dpending on your individual case a chemical peel could offer relief in one week.