Acne. Your doctor may be able to help you with prescribing a antibiotic/acne medicated body wash as well as retin-a. This will help address the 3 ways that lead to acne - oil production, clogged pores, and bacteria. You may also try to wear lose fitting clothing particularly during activities with high perspiration.
Light Chemical Peel. You can have jessner's peel or some mild acid peel on your back and shoulder along with oral antibiotics and retinol or tretinoin. You will see your skin peeling 4-6 days after and it is helping to have less break out on your back.
Laser or Accutane. You can settle for over the counter topical creams that dry your skin and give temporary relief. But then you also need to buy a whole lot of cream to cover your shoulders and back. A better solution would be a series of laser procedures to clear the acne away. The most effective and permanent treatment, however, is Accutane (retinoid). Given in gradual doses it works wonders, on the back/ face.
See a dermatologist . There are a variety of forms and therefor numerous treatments. Not everyone responds the same and treatment may need to be adjusted. See a dermatologist.