Can anyone help on my story, ive got the general idea, just need a bit of help?

So far - I have a fourteen/fifteen year old girl and I know that she used to be in a carehome, she now lives with a new foster mum and its her birthday at the start of the story. Do you think maybe it should be about her getting the present she wants (a horse) for her birthday, or falling in love with her new school teacher?? or maybe it should be about both?? and what can have happened to her when she was younger to get her in a care home ?? please help, thankyouuu xxxxx