Well, I'd say you've got your hand full there. Should keep you busy for at least one winter.
First thing is always to protect what's left against the elements. It appears you are missing some windows, so quick like a bunny get something covering that up. Then start taking it apart, I figure.
If it has sat for as long as it appears to have, you will need a completely new floor, as a starter. Start searching/googling for shell-off refurbs, you'll find many good histories here.