Whatever is mandatory for the story and gameplay. Like if you're restricted to the house what is suppose to happen in this area, are we just exploring? What are we exploring for? Are we a little kid, who can't go outside? Or is it like a megaton blizzard outside, and where an adult who can't leave?
What are people going to learn in the prologue? Are there any special controls besides the basics, that exist(If you're just telling people the basics ie arrow keys = move and enter = talk, IMO that should be introduced in either a splash screen or an event before the title screen, as if you are assuming the player has no Idea of how to use the controls at all, that's when you should tell them). Should the player know that they should explore as they're might be hidden items or lore? In terms of backstory, who's backstory is this? The child's, one of their parent's, someone else's. Figure that out and then ask yourself, what do you want to say or show. Do you want to show a happy family, right before the villain comes and kills everyone but the lone survivor. Or are you showing a happy childhood in contrast with a bleak adulthood. It's all up to you and the story you wish to tell.
