Menarche
A Menarche can change a scary, unpleasant experience into a ritual that makes the girl feel special. The first thing to remember: the focus is the girl--she must be the center of attention, and she must be consulted on what and who she wants for this ritual--emphasis should be placed on her being a woman, women's mysteries, women's responsibilities. Chose the first full moon after the girl's first period for the ritual (that or quarter moon)--or when the girl wants it.
You want to invite any adult women the girl wants, the girl's Goddess-Mother, and any friends of hers that are also of age. Decorate depending on the girl's taste, with an emphasis on moon decorations/party decorations. Get a cake, candles--make it festive and special! Incense/candles, again, the girl's choice--but white/red are the traditional colors.
To give the men something to do, you can have them help in decorating, and making up the pot-luck dinner, tea, or picnic lunch for after the ritual (I like this idea because it allows Dad and friends to give the girl a gift even though they're not going to be there). Otherwise, tell dad and friends to go to a sports bar for the evening.
All the women invited should bring gifts (gifts related to womanhood, not menstruation gifts!)--small gifts, magical, adult gifts (like a mirror, a special herbal tea, a diary, or a tarot deck). Mom, buy the girl (and or let her pick out) a moon, crystal, goddess or pentagram pendent for herself. Place that on the altar. (A new dress, ritual robe, etc., might be in order for the occasion as well). If the girl is 10 or 11, face painting or dressing up might be a part of this celebration.
The ritual can take place wherever the girl likes--it's her show. One menarche I heard about was a hot tub party, another occurred on the beach, another up along a hiking trail in the woods. Near water is a good idea.
The night or that morning before, it's time to have that mother-daughter chat. I know she already knows it all, but tell her again just the same. Let her know where the contraceptives are, let her know if she can have sex in your home--what your rules are, how you feel about it (BE HONEST), let her know about the dangers of unprotected sex, including pregnancy--remind her that she's not old enough to drive, vote or leave home, but she is old enough to have a baby.
Now that she is a woman, she will have new, adult privileges, but she will also have new and very serious responsibilities--not getting pregnant until she's wants to, until she's ready, is one of them. Let her know that you're there for her, that she can come to you.
After that, the girl might like to meditate till the ritual, or go through a self-cleaning ritual.
After the circle is cast (an all goddess circle--goddesses for the quarters--emphasizing the goddess), each woman, one by one, should welcome the girl into womanhood and hand her their little gift. Each gift should be opened and talked about.
After the gift-giving, the girl's new pendant should be passed round, and each woman should talk about her first time, offer advise, a secret, a funny story--words short and sweet, upbeat; then they should bless the pendant. Once the pendant has been passed round, Mom, who should be last, says her thing, and then hangs it round the girl's neck.
The HPS should then question the girl as to whether she understands the responsibilities of Womanhood. After the girl answers, the HPS should then present the girl to the Goddess as a new woman, a reflection of the goddess. At that point, the girl should say something.
Other things you can do: A Tarot reading for the girl or a scrying, dancing, singing, storytelling. End with a very positive cone of power--celebrating the girl's womanhood (and then have that picnic).