If your complexion is naturally dark, then it is usually the result of excess melanin (skin pigment) in your body. There are several possible reasons for you to have developed a dark complexion such as heredity, vitamin deficiencies, malnutrition, hyper pigmentation- where the skin produces melanin at an accelerated rate, skin disorders that can leave the skin patchy, thick and dark or even too much exposure to the sun. The UV (ultra violet) rays of the sun tend to penetrate the skin and the skin tries to block it by producing excess melanin to protect itself, but adversely causing the skin to darken.
Dark skin cannot immediately be lightened and requires regular care and attention. If you are out in the sun often, you could use a sunscreen lotion of an spf above 30. Apply this on all exposed parts of your body and wear clothes that cover up as much of the skin as possible. While this may be impossible in summer, it does go a long way in keeping harmful UV rays from directly affecting your skin. There are a few remedies to lighten your skin tone which you can practice easily at home. Slice a raw potato into thin slices and place over the face to remove patches and other blemishes. A mixture of curd, oatmeal and tomato juice applied on the face for half an hour and then washed off with cold water lessens the tan and lightens the skin. Lime juice which functions as a natural bleach can be mixed with a little turmeric and then applied onto the face and neck. Grate a tomato and add a few drops of lime juice into it. Apply it onto the face for about 20 minutes and then wash it off with cold water. This could be repeated for about 3 weeks for the results to show.
Apart from these it is important to maintain a healthy balanced diet which will lend your skin a natural glow. Make sure to eat fresh fruits, vegetables and include pulses and cereals in your diet. Water has the ability to wash out toxins from the body and it is hence necessary to drink at least 4-5 liters of water a day. In some cases a dark complexion may even be an indication of intestinal, liver or adrenal disorders in which case it would be advisable to first consult a dermatologist and then your physician.