It could say all sorts of things. Maybe it warns of danger or reveals a hidden secret. But without more context, it's hard to say for sure.
That depends on the specific ghost story. The note might describe the ghost's past, give instructions to escape its curse, or just contain some creepy message to scare the characters. Without knowing the story, it's impossible to tell exactly what it says.
I have no idea. You haven't given any details about the note or the ghost story.