What Do You Think of My Book?

My idea: 19 year old Mandy Churchill is 5'1'' with ice-blue eyes and ginger-colored hair. She has a weird/creepy day that includes her running into some weird old lady that warns her to stay home and keep a close eye on any close family or relatives. She brushes this off and goes to work (I haven't decided what her occupation is yet). After a day of her job, she heads out to hail a cab. When she gets in the taxt, her cellphone rings. She doesn't recognize the number but answers it anyway. A real gravely scratchy male voice comes on the phone, screaming in the background. He tells her she has to kill the taxi driver and steal the taxi and drive over and meet him some place (I haven't decide where yet). She almost hangs up but then he puts her 8 year old sister, Kim, on the phone. She is crying and asking Mandy to come get her. Mysterious man comes back on and tells her of she calls the police, he will kill Kim and also the rest of Mandy's family. She is really scared but kills the taxi driver anyway ( I haven't decided how to she can kill him yet). She puts his body in the back seat and drives to the place he told her to. When she gets there, no one else is there but she finds a note on the ground telling her to go somewhere else and kill someone else. The whole story goes on like that until he actually comes out and it turns out the guy was her ex-boyfriend who she broke up with because he was so violent. He wanted to kill Kim and than Mandy but Mandy got a gun at some point during the story and shoots him. I haven't quite decided the rest of the ending after that. Lemme know how you like it and if you have any ideas! Thanks.