Your teen girls period
One thing that no one really prepared me for was that they first year or so of a girls period would be very irregular. I didn't recall from my teen years, in fact I probably paid no attention to it. While some may be regular from the start, it is much more common for it to take awhile to even out.
There is no reason to be concerned about this unless it has been closer to two years and it has not evened out. At that point you should speak to the pediatrician to decide if there is any reason to worry. Most likely there isn't unless other symptoms are present.
Do you recall being regular right away or did it take awhile to even out?