I have dark spots on my face after i get a pimple. when i have a pimple and it dissapears it leaves a scar on my chin-under my chin. what should i do?

You have PIH: It sounds like you have post-inflammatory hyper -pigmentation. Try to minimize any inflammation such as pimples. Gentle cleansing, use of benzoyl peroxide products, plenty of sleep and a good diet and exercise will help you keep pimples under control. If you get one, do not squeeze it! you might want to consult a dermatologist for help in control and fading these spots.