12 year old started period

I have custody of my 12 year old niece and she started her period today. I have no children of my own and only use this app to track my periods. How the heck do I talk to her about this?? We've had the talk before buy nkt that it's actually happening I'm freaking out more then she is! Hahaha do any of you have teens that started her period? Should she use pads first? Inform me!!