The girl's granddad died, and the family is in his funeral, girl never really knew her granddad. Family never really talked about him. In his will, he gave his only possession: a moon stone for the girl. Girl always bring stone around with her.
After her granddad died, girl woke up one morning to find herself able to hear people's thoughts. Have a freak out. After she calms back down, she convinced herself that she will find the root of powers after school in the basement of her house, as her grandmother collected a lot of old books about supernatural stuff.
She kept hearing people's thoughts and secrets they are keeping. She found herself taking note of those secrets. When she was talking to her friends, she found out her friends never really liked her, got in a fight and left home early.
She got scolded by her parents and went to room. After her parents went back to work, she sneaked into the basement and searched around the dusty bookshelves.
Found out that the moon stone is something helps people inherit the ability. The moon stone has been passed down for many generations. Though her granddad gave her the ability because she is the only one who can handle the burden and side effects of the ability.
She got used to the ability and helps people with it.