Without any more details other than a 'dark spot', the best advice would be to visit a dermatologist (skin doctor).
They specialize in all manner of skin problems
and can offer you suggestions and treatment for your spot. If the edges are abnormal, if the borders are irregular, if the color is uneven or if the diameter is >6mm, it could be melanoma
. In this case, see a doctor now. Melanoma kills.
Answer assisted by Cole Livingston, Medical Student