Here is the plot summary from Wikipedia:
Gilly Hopkins is the meanest girl in town: she has a birth mother she has never seen and has moved from one foster home to another. Learning that she will be sent to a new home, she wastes no time stirring up trouble. Then she meets a woman named Trotter, who wants to be the mother Gilly needs. With Gilly posing a challenge, Trotter must prove a good relationship will benefit both of them.