During the years of adolescence, girls develop into women. Physically, your daughter will become taller and put on more weight due to both the increase in height and an increase in fat deposits. Secondary sex characteristics develop, making it a lot less likely that she'll be mistaken for a boy anymore; she'll start getting her monthly period, and her reasoning skills will develop. Other signs that your daughter is going through adolescence include acne, growth of pubic and underarm hair, increased sweating and body odor, and increased moodiness.
Take your daughter to a dermatologist if over-the-counter acne ointments don't keep her acne satisfactorily under control; make sure that you're well-stocked with sanitary napkins (pads) and deodorant; and expect that your daughter will spend hours in front of the mirror examining her hair, her skin, and how her clothes look on her. Many girls decide that they're too fat and go on crash diets or develop eating disorders, so you want to keep an eye on your daughter's weight to make sure she's not overdoing a good thing (wanting to be in healthy shape).
Teens also develop a self-consciousness during these years, which makes them feel as if everyone around them is noticing what they wear and judging them for it. In addition, your teen may become judgmental of the adults around her and point out when she disagrees with your system of doing things. Your daughter may become idealistic and spiritual or she may devote a lot of time to a cause like Save the Whales or tell you she wants to join the Peace Corps. Try not to take your daughter's preaching personally or feel that she's rejecting how you raised her. As long as her safety isn't at stake, you can probably ignore a strange haircut or a passing fad.