The skin in some parts of my body is darker than the rest?

I'm a 17 year old guy.I'm Indian and i'm brown.The thing is certain areas of my body are darker than the others.My elbows,knees,hips and groin area are relatively much darker compared to other parts.So what do i do about it?Also I've heard that lemon helps to reduce skin complexion,but i guess it would be kinda awkward to like rub a lemon on my butt.So any ideas?