from what I know, vitamin c is the one with the effect of lightening skin. the quickest way is by getting a shot of vitamin c at your local beauty center. not with a cream. a friend of mine does it once a month. her husband is the consultant (him being a physician) at a well known beauty center here.
but you must know that pigmentation of the skin can't change drastically. if you were born with dark skin, I suggest you accept and love the way you are. some people consider girls with dark skin are exotic :)
but if you really want to, then just try the method above. good luck :)