It's important for women (and teenage girls) to keep track of the menstrual cycle in order to be able to plan their lives accordingly. For example, if you know you'll be getting your period next week, you might choose to go swimming this week instead. In addition, if you keep track of your normal monthly cycle, you'll be able to quickly catch any abnormal bleeding and bring it to your doctor's attention. Yet another reason to maintain a menstrual cycle is to accurately date a pregnancy (is it the seventh week or 12th week?) and predict a baby's due date. If you're from the low-tech generation, you can go to your local stationery store and pick up a regular month-at-a-glance pocket calendar and explain to your daughter how to mark down in one of the little boxes when her period first begins and how to keep track of the length of each cycle.
If you and your daughter prefer a high tech way of tracking your cycle, you can search for MyMonthlyCycles.com, which helps you track your menstrual cycle, figure out when you ovulate, predict when your next period will start, track results for breast exams, keep track of PMS and perimenopause symptoms, and maintain a history of your cycles that you can print out and show your doctor. If you want, you can set it up to send you reminders to do a breast exam X number of days after your period or to call for a gynecological appointment. Lady Timer is a similar free service that also allows you to track your period, and it can send text alerts to your cell phone when you're about to get your period.
Yet other online menstrual calendars, such as FertilityFriend and Ovulation Calendar, work on the same principle but they're set up for women who are trying to become pregnant, and so they focus on finding the most fertile days.