Of all the changes in store for a pubescent girl, acne may be the only one that doesn't serve some greater purpose. Acne blemishes are caused by the blocking and inflammation of pores in the skin that produce sebum. Sebum is an oil that helps moisten the skin and keeps bacteria from getting in the body. Sebum is produced in the body and then travels up the follicle to the skin's surface, where it mixes with salts, dead skin cells and other matter to form a type of protective coating.
Unfortunately, however, when sebum mixes dead skin cells while still inside the follicle, it can block off the follicle, get infected by bacteria (Propionibacterium acnes, or P. acnes for short)and become inflamed. End result: zit.
It's a hard enough time in a girl's life: Her entire body is changing both inside and out, she feels like she's especially under the microscope, and now her skin is rebelling against her.
Girls tend to have fewer problems with acne than boys do during puberty. However, after puberty and throughout adulthood, girls are more commonly afflicted with acne (perhaps making pubescent acne that much more frustrating).
The best ways to prevent acne during puberty (and beyond) are gently washing your face, completely (without hard scrubbing!) removing makeup each evening and showering after working up a sweat. Also, don't pick at your face or touch it more than necessary, as this will agitate the skin and lead to more breakouts.
If any particular food seems to cause acne outbreaks, it is best to just trust your own judgment and avoid it for a while to see if that helps your skin. Remember: There is no single cause of acne, and likewise you often require a mix of preventive measures to stop it.
To treat acne, try products containing benzoyl peroxide or salicyclic acid. These substances dry out skin, kill the bacteria and help the skin shed its dead skin cells. Start off with small amounts of these products to give your skin the chance to get used to the medications. (Need to learn more about acne? Try How Acne Works.)