The skin color is determined by genetic factors and environmental factors. That means if you are fair since birth and if your parents are fair then the chances of you staying fair are more unless you get tanned.
Getting tanned is a example for environmental factor.
So if you have been dark skinned, then nothing much can be done.
Some creams are available which contain steroids and they claim that they make a person fair. But trust me steroids should never be used on face for fairness unless otherwise indicated by a dermatologist.
Because it will cause your skin to thin down, excess hair growth on skin, excess pimples when you stop the creams.
Consult a dermatologist for further help.