Dear K.,
There's really no way of knowing when she'll start. Girls start at such different ages. I'd had so many problems with endometriosis and my daughter was really afraid she'd have the same problems. She was very surprised when she started her period at 13 because she'd never had any cramps or anything. It just happened one day. And, fortunately, she has never had any issues with pain or anything. When she was about 15, however, the curse of PMS hit our house like a tornado. I knew everytime she was getting ready to start because she would get mean and viscious and slam doors. Once she started, she was back to normal.
Just be open with your daughter. Make sure you have pads at home, just in case. Make sure she knows that all girls are different and if she starts her first period on the 10th, it might not come on the 10th the next month, but that's okay. Very few little girls, including my daughter, had "regular" periods right at the start. I got my daughter a really cute little zippered coin purse that was the perfect size to keep a couple of pads in for her backpack in case she started at school.
I had mentioned to another mom that when I was 12, I got an ear infection. I'd never had one before and I was pretty sick. My mom sent my dad to the store to get a heating pad.
He came back with a box of sanitary napkins and a belt. God bless him. He must have been thinking, "Hmmmm. First ear infection. She must be getting ready to start her period." :)
I didn't start for 3 more years and I may be old, but for heaven's sake, they DID have pads that didn't require a "sanitary belt".
So...just be prepared and know it will happen when it happens. There's no point in worrying about it every day because it may not happen for another year. Or longer. Our bodies do things on their own time.
Best wishes!