1] Take some Tulsi [Holy Basil Leaves] and mix them in water and crush to make a paste. Apply this paste on the face; especially on the dark spots. When the paste dries, sprinkle water over the face and softly remove the Tulsi Paste.
2] Rub the inside of the skin of Sour Lime over the face and after about 5 minutes apply a paste of Gram Flour over it. Then after about 15 minutes, sprinkle water and remove the paste.
You can also have a look at these earlier articles and decide which one suits you best.
Traditional Indian Remedy for Dark Spots
Remedies for Removing Dark Spots from Skin