When you first have your period, you may find you have another in just a few weeks or you might not have one for several months, it can start off as irregular and then become regular or it can equally go the other way. An unpredictable cycle is not necessarily a sign that something is wrong.
You can keep a menstrual calendar in your diary and measure the time between the first day of your menstrual period and the next. Do not be surprised, as this can vary between 19 and 35 days, apparently only 14% of females actually have a 28 day cycle.
Taking birth control pills can be a reason for a more predictable cycle but you will eventually be able to tell when your period is due by watching your body. Some
girls will get cramping or skin blemishes, while others will get sore breasts, it is a good idea to keep tampons or pads handy in case you start unexpectedly. The period may last between 2 - 7 days and during this time your flow may vary, so a light day doesn't necessarily mean your period is finished.