The Clue period tracker allows you to enter details about your cycle so that you can easily see when your next period is due. It is free on Android and IOS. Unlike many of the other period tracker apps it is not bright pink, nor does it look like a kindergarten game. Clue also has a website with many articles and videos, some of them specifically for pre-teens and teens. I particularly enjoyed Erica Avey's article, Advice to my 13 Year Old Self.