Welcome to healthcare magic,
What you are suffering from is called post inflammatory pigmentation
due to acne. Also your skin will be more sensitive to sunlight because of acne treatment causing sun induced tanning
You need to wash face twice daily with a face wash ideally a one containing salicylic acid
which will decrease pimples, oiliness and will help in pigmentation as well. Apply a gel based sunscreen , not cream or lotion during morning hours and reapply every 3 hours.
You may also approach a dermatologist
practicing cosmetic dermatology for a few sittings of chemical peel for faster improvement.
Have a diet rich in vitamin A
for eg: carrots, pumpkin, pappaya, tomatoes, Beetroot anything with lot of orange or yellow colour. Drink plenty of water and wash face regularly. Avoid milk and milk products like milk shakes, cheese, chocolates, and sugary foods as much as possible.
Hope this helped,