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. Many treatment options are available for Acne scars and pigmentation which the pimples leave behind after healing. Some creams are available but they are not very effective. Chemical peels can help you reduce the scars and pigmentation but the best treatment is carbon dioxide laser.