Supplies should include some sanitary napkins / pads / panty liners. Not to put too fine a point on things, but please don't cheap out on these items. A teen or pre-teen girl would be mortified to have something about half an inch thick in her panties. They are hugely self-conscious about other people noticing ill-fitting undergarments / bulges of pads beneath their regular clothing. I don't know that we ever get over that self-consciousness - up until my very last period prior to my hysterectomy, I would have died of embarrassment if my pad bulged on my behind and showed through my trousers, just as much as I would have been if I'd had a feminine hygiene "accident".
The "Always" brand has what I consider to be the Cadillac of pads - "Infinity" is extremely thin and is made of a material called "Infinicel" that is tremendously absorbent, along with the traditional Always "wings" that wrap around the panty's crotch area for extra protection; some also come with a moistened sanitary wipe so ladies can freshen themselves with every pad change. They are a lot smaller than traditional pads too, and therefore more discreet in a young lady's purse. (Yes, she should start carrying a purse so she's not caught off guard.) The more discreet the supplies, the less self-conscious she will feel. I will never forget how humiliated I felt when - almost 40 years ago - a boy in my class somehow figured out that I was on my period and started telling jokes about how vampires made tea with used tampons.
Something else that will be handy are pain relievers. You can get some specifically for menstrual cramps, like Pamprin or Midol, which contain both a pain reliever and a mild muscle relaxant for cramps. In a pinch, Motrin or Advil will be effective, provided she isn't allergic or sensitive to ASA or NSAIDs. Other comfort measures would include a hot water bottle for cramps or backache, and a properly fitting sports bra for breast tenderness.
Now, while you don't want her to get the idea that she can get out of doing important things like homework or normal chores on a regular basis when she has her period, she may have a couple of days where she gets headaches or is moody or irritable or in pain. She may not be on her period, but experiencing PMS symptoms - so just be aware and sensitive to her moods at those times. She may get food cravings, which can be indulged in moderation - studies have shown that a woman's metabolism increases just before and during her period, and cravings for food with higher fat and sugar content (chocolate, anyone?) are the body's way of meeting that need. Make sure she has a comfy pair of pajamas and slippers and fluffy bathrobe, so she can feel cozy when she's feeling poorly with a visit from "Aunt Flo".
It's also a good idea to teach her to mark the days of her period on a calendar. When she gets older, or if she has menstrual irregularities that warrant medical attention, she will have an accurate record for the doctor. It isn't unusual for a girl's periods to be irregular the first year or so following menarche, so you can reassure her that her body should sort things out, and also that you'll have supplies ready for her so she won't be caught unprepared.
Good luck to you both!