Please Help How to start talking with a girl on facebook if you do not know her?

Anyone can (and will) give you a "proper" answer, but it will be based on what they would do or using just common sense, what someone can say to "break the ice" I would suggest you to dig deep in yourself and imagine what would you say to a live girl in front of you after saying "Hi" ... now, this would make it easier because you'll not be (or shouldn't be) nervous if she's very attractive and you are shy or whatever ... Just talk (or write instead) to her in your own words, and what you would think that can keep a conversation alive. Good luck.