Melanin is the colouring pigment found in the skin, and darkens through exposure to the sun - this is how you tan. The amount of melanin present in your skin depends to an extent on how much sun exposure a population has received in the past, but also on individual genetics. Even within races there are fair skinned and dark skinned.
In order to reduce the amount of melanin, and in turn lighten your skin, there are some popular methods around which work to differing degrees:
Potato. Yep, really! Rub a slice of potato daily on your skin to reduce the appearance of dark, pigmented areas.
Citrus fruits. Lemon juice is a natural (and in comparison harmless) bleaching agent. Apply daily with a couple of teaspoons of honey, for it's natural moisturising properties.
Turmeric. A common ingredient in natural facials (mixed with milk) to smooth the skin. Combine with lemon juice to help reduce the effects of tanning.