This is how it happens. The skin is constantly renewing itself and under normal conditions the old skin cells will shed. But sometimes these cells will stick together and they plug the hair follicle forming a comedone. Open comedones are blackheads and closed comedones are whiteheads. When a closed comedone ruptures you get a pimple or zit.
Pimples vary in size and appearance.
They start out as papules which are small reddish bumps. They turn into pustules when they come to a head. And there ones that get inflamed and they are called deep cysts which contain blood and pus and can cause scarring. For mild acne using an over the counter topical solution will help to alleviate it. Look for a gel or soap formulated with benzoyl peroxide which kills P.ances, the bacteria that causes acne.