I’m very new to Ember as well, so hope I’m correct, but Ember is a client-side framework so you will not use Ember to create a REST API as such, you will only use Ember (or Ember-Data) to consume a REST API created in Rails or PHP or some other server-side technology.
See this post for a full-stack solution that might help with creating the REST API side of things on the server: Ember + PHP with JSON API