Pubert in girl starts at the age of 10-14. During these days hypothalamus secretes hormones which induce secretion of stimulating follicle hormone (FSH) and Luteinizing hormone (LH) from the pituitary.
Under the hormonal influence, Ovary starts producing Oestrogen which leads to puberty.
These changes include
Enlargement of breast and darkening of the areola.
Initiation of menstruation.
Hair growth in the armpit and public areas.
Increase in height.
For detailed information visit Palo Alto Medical Foundation page.