My daughter was only 10 when she got her 1st period and I taught her to use tampons right away! She started out with the Lites and a pad, and "graduated" to regular/plus tampons as she got more used to it. Inserting the tampon standing up and bending forward can be easier than sitting down, since sitting kinda "scrunches" everything down and makes the vagina tighter. Also, putting a bit of Vaseline on the tip of the tampn helps it go in more smoothly. These tricks, along with lots of encouragement and patience, should help. Also, i made sure to explain the dangers of Toxic Shock Syndrome to my daughter, and tried to "keep track" of her tampon changing until I felt confident that she wouldn't forget to change it.
Ehh I would wait a little longer I mean I tried tampons when I was twelve nd they just didnt work out for me but now i'm almost fifteen and i just started using them. Ask her what she wants to do..
It might actually be very difficult for a 12 year old to use tampons successfully. If she does, you will want to make sure she has the ultra lites to start out with. Why not just use pads? The new ultra thin ones feel like they're barely even there! :)
You can show her the directions that the box gives, or show her with the tampon how to unwrap it and put it in(without actually putting it in). I'm sure she will be able to figure it out after a couple tries, or maybe get it on the first try. Good luck