Hello..
There are multiple reasons for dark circles and hyperpigmentation on neck. Best is to find a reason and treat the cause ... That should solve your problem.
For back acne , depending on severity treatment plan is decided .. whether only creams are sufficient or it needs oral medications as well
Next Steps
Visit a dermatologist so that he/she may evaluate the reason behind it
Health Tips
Drink plenty of water , eat lots of fruit and high fibre diet , with proper night sleep and avoid stress