You can start by reading what have worked for others.
On the top of the list is diet.
Underlying cause of Acne are usually Antibiotics , artificial hormones and poor diet (processed foods, sugars).
Even Antibiotics and artificial hormones taken by mother can be the underlying cause of Acne in the child, even if child have never taken Antibiotics or artificial hormones or other medications.
Those things disrupt liver function, natural function of hormonal glands and it destroys intestinal flora.