The other thing to remember when you say "I don't have a date for my periods," is that you should not expect it to come on a given day of the calendar month, since when you think about it, the calendar is unstable in that regard. February only has 28 days, May has 31. Nobody can say "My period always comes on the 25th" or any given numeric calendar day because they would have to have periods that could read a calendar and know when it was April, June or September (or February) and when it was a 31-day month instead.
You might get some grid paper and begin a calendar with the first day of your next period. Write the days of menstrual bleeding in red and the rest of the days in blue or black. Then when the next period begins, start a new line under the first one. After three or four months, you should begin to see a bar graph that tells you how many days on average your cycle is, and how long your flow usually is. It should start to help you project forward when to expect your next one.
Good luck! Frustrating, I know, but it will get easier.
That is very common for girls of your age. Many are irregular and get more stable as time goes on. Nothing to worry about but know it's a pain. If it doesn't get regular in the next couple of years, talk to your doctor about it. good luck