When you get acne on back and shoulders it is generally hormonal and certain long term medication needs to be prescribed which requires follow up So you need to consult dermatologist for that and not self medicate or half hearted treatments won’t work. For time being you can use brevoxyl creamy wash on the area to reduce seborrhoea. But see a dermatologist as early as possible to prevent scarring.

Hi Share pictures of your back to identify the condition and accordingly treatment can be advised. Effective medicines are available