There are many reasons why your skin would turn dark. Sun tan, overweight, pigmentation disorders like acanthosis or melasma, nutritional deficiency etc are some conditions. Unless someone examines you, we cannot know the cause. Unless we find out the cause it is difficult and pointless to treat. Please send photos with detailed history in private chat for proper treatment. Use a good sunscreen and start exercising. Take lots of fruits and green vegetables in your diet.