How To Cure Dark Patchy Skin On The Inner Thighs When Compared To Body?

Hello Doctor, My skin is very dark near my butt and inner thighs compared to my rest of the body. Also, the part behind my knees and such. I could also see lot of dead skins on my butt and black marks on my waist. i think this dark mark on my waist is because of wearing tight pants. Same goes for my armpits. How can i get rid of this dark skin? Is it some illness or such?