How do I treat a staph infection on my butt?

Antibiotics. Depending what the staph is sensitive to various antibiotics can be used. Surgical soap may be helpful in decreasing the bacteria to decrease chances reinfection.
First confirm its . staph. If it is then you need prescription strength Mupirocin, and likely an oral antibiotic like doxycycline or Bactrim (sulfamethoxazole and trimethoprim)