ANDY'S ANSWER: When your breasts are getting bigger and you start growing hair under your underarms and in the vagina area, you are getting close to starting your period.
Some girls get a white mucus discharge from their vagina up to 6 months in advance of starting their periods. Although these are signs, you'll never know the exact time your period will come until it does. Just be prepared by carrying a pad with you at all times.
Remember though, even once your period starts, it can take years to become regular.