How to message a girl I like on FB?

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So I was just curious. I like this girl and I suspect she likes me back. The problem is that I was a senior in school and I just graduated so I could never actually have any time to get to know her, so I added her on FB. Though I don't know how to get the ball rolling with the greeting part. I never talked to her in High school so we never talked in person. I want to get to know her and I want to send her a PM but I don't know how to start it off. Should I start it off with a "whats up" or a "hello"?

I am trying to mainly get female advise here but I dont talk to a lot of girls so I can't really get female advise.
I also never had a girlfriends, for the record.
girlfriend*
Also I suspect she likes me back because she gives me lots of looks.
I just graduated highschool recently so it's not like ages have gone by.