My butt has darker skin than the whole of my body, and rough skin.. How can I get rid of it?

Buttock discoloration can be improved with topical creams, including Melarase AM and Melarase PM used twice daily on the affected areas. Raffy Karamanoukian MD FACS

Dark skin is prone to hyperpigmentation.
Chronic moisture can cause skin irritation and result in hyperpigmentation of the inner thighs , posterior thighs and knees, underarms, buttocks and back. 
Before you spend $$ on peels or IPL, try these topicals.
This is called post-inflammatory hyperpigmentation (PIH). 
I recommend Melarase AM and Melarase PM creams from Kare Skin as well as a broad spectrum sunblock like Spectrase. Bothe are from Kare Skin. Scler-x post inflammatory hyperigmentation relief complex is a great supplement to take as well. See link below. 
Spectrase is a wide spectrum sunblock SPF 50+that is also recommended to prevent ongoing inflammation.