It is important to present the changes of puberty and menstruation in a calm and clear manner, and in a positive way. You do not want to scare young girls, and you want them to see it as a natural part of growing up.
Fortunately, there are a lot of good books available that can help you get started with it. One of my favorites is The Period Book: Everything You Don't Want to Ask, But Need to Know by Karen Graevelle. This is written for girls, and you can read it together as a way to begin discussing the topic.
Other good books are: What's Happening to my Body Book for Girls (there is one for boys too), by Linda Maderas, and It's Perfectly Normal, by Robie Harris. There are a lot of other books on puberty, and you should visit your local library and scan through a few to see which ones appeal to you.
As far as underarm hair goes, I recommend leaving it alone for as long as possible. In the first year or so of puberty, it is often not very noticeable. For girls with very thick underarm hair, or problems with excessive underarm odor, removing the hair may be helpful. I would not use hair removal products, as they can be very irritating to the skin. It would be preferable to clip the hair or use a gentle razor, with supervision by an adult.
