You are suffering from what Dermatologist call tropical acne. It comes on the back and shoulder area as red boils which then subside after few days with pigmentation. The treatment is not very complicated. It can be treated with tablets, creams, and soap. Maintanence treatment may be required to cover for recurrence. Please dont  apply oil on your head. Please send photos for proper examination and detailed prescription.

Greetings. Pimples may often leave spots especially if not treated or if we burst pimples or pick them. Marks can also occur after a few procedures or after injuries etc. You'll need skin lightening creams for a specified duration to reduce the marks, and this could also be dealt with chemical peeling. Also you must use a sunscreen to prevent darkening of the spots. For scars, creams may not work n you may need procedures like microneedling, microneedling rf, fractional lasers, prp or subscision or a combination of these. See a dermatologist near you and discuss the same with him. Hope this helps. (Caution: do not be lured by unrealistic claims and offers made by quacks who claim to be specialists in trichology n cosmetic sciences or even saloons. Make sure u are treated by a qualified dermatologist). Warm regards, Dr Govind Suresh Mittal, Bengaluru.