How can a girl be prepared for her first period, especially if her father is raising her alone?

Explain it before it happens — usually somewhere between nine and twelve years old — in calm, practical language and without embarrassment.
ʿAisha r.a. praised the women of the Ansar because shyness did not stop them from asking about religion, including menstruation (Sahih Muslim, Kitab al-Haid). Things to explain: menstruation is something Allah has decreed for women and is not dirty in a shameful sense; how to wear and change sanitary pads; the signs that a period has started and ended; that prayer and fasting are not performed during menstruation and missed fasts must be made up; and how to take the obligatory bath. Prepare an "emergency kit" for her school bag. A single father can ask an aunt, grandmother or trusted female teacher to help, while still showing his daughter she can ask him. See Fiqh of menstruation and Single parents.