Elsewhere

Lizzie is 15, almost 16. She's a nice girl, does well at school, has a best friend, loving parents and a younger brother.

She's also dead.

Lizzie awakes on a boat and does not realize she is actually dead. The boat is taking them to a place called Elsewhere, where the dead go.

While there, she will age backward. Find a grandmother she never met. Get a job at a place that helps recently arrived previously-dead dogs find new owners. She also is able to speak the language of dogs, which helps considerably.

Oh, yes, she'll also have a dog of her own.