Yeast is a common organism that lives in the vaginas of girls and women. It's the same bug that causes diaper rash in babies! So yeast is around even if you haven't been sexually active.
Infections can occur when more yeast than usual grows in the vagina, which can be caused by a variety of factors. Infections are usually treated with creams or suppositories that you can get "over the counter," or without a prescription. These are put into the vagina (each medication has specific instructions on how to use it, so read the label).
The most common symptoms of a yeast infection are white discharge that looks like cottage cheese and itching inside the vagina and on the vulva (lips) outside. If your symptoms don't improve after your first over-the-counter treatment, you could try a different over-the-counter product. If symptoms still remain after a second course of treatment, then you should see a doctor to get checked out.
To prevent yeast infections, make sure to wear loose-fitting underwear with a cotton crotch so the vulva doesn't get too hot and moist, which can cause yeast to grow.