Reducing or eliminating dairy and sugar/carbs can help your skin start healing from day one.
Be careful to eat enough during this time and don't go off carbs and sugar cold turkey, but let it happen gradually.
I have personally found this diet to be the most effective for reducing acne, but it is also difficult.
Make sure to eat plenty of fish (like salmon) and meat if you are not vegan. Also do not be afraid to eat carbs--try to start with eating less than 100g of carbs a day.
Increase the amount of blueberries, strawberries, and spinach that you eat.
Let me know if there are any questions in the comments.