Be prepared to get blood on some of your panties, forever ruining them. Also, tampons may become your best friends, but until they do, when you need extra coverage just take one pad and stick it onto the end of another pad to make a super long pad :) I do that when it's bedtime so there's no leakage. If you decide to use tampons, change them roughly every six hours; I recommend tampax pearl. You might also have mood swings eventually, which, as you get older, could include bursting into tears everytime a gerber baby commercial comes on, or spending inappropriate amounts of time watching Say Yes to the Dress. For cramps, take Aleve or Advil or Mydol or whatever, and exercise. Don't deny the chocolate cravings, of course. above all, remember that it'll be over soon, that you're not alone, and that a period's a good thing for you right now, because it means you aren't preggers :)
Mark it on the calendar. At your age, it might not be predictable every month. It's good to keep track of when you stop and start each month. I used to put a sad face on my start day and a happy face on my end day.