How do i know if it is cancer ? what do i do if it is ? what kind of a doctor do you go to see for it ?

It is VERY VERY unlikely you have skin cancer at your age, especially in a place that does not usually see the light of day. Generally, skin cancer spots are slightly bumpy and a little raised. My guess would be its simply a mole or a birthmark. If, however, you notice it changes in size you may want to first see a gyno, then they can direct you to to proper doc.