Most girls in the U.S. start menstruating between 12 and 13, but it's normal to start having periods as early as 10 or as late as 15. It's never too early to talk to your daughter about what she can expect from puberty, so go ahead and strike up a dialogue as soon as you feel she's old enough to understand and seems open to it. Menstruation tends to begin about two years after a girl's breasts first start to grow, so when you notice your daughter developing breasts, it's definitely time to start chatting.
Your daughter's first few periods will probably be irregular and painless, since menstrual cramps usually don't begin until a girl actually starts ovulating, which is usually 12 to 18 months after her first menstruation. Once these cramps start, over-the-counter pain relievers like naproxen (Aleve) or ibuprofen (Motrin or Advil) are the best medicine, since unlike acetaminophen (Tylenol), they specifically address muscle contractions, which is the source of menstrual pain.