How to talk to this girl on facebook?

[inside and out]

We are both 21 years old and we used to go to high school together. During High school I had a big crush on her, but never did anything about it. We are facebook friends (but have never talked since high school). She is beautiful and cute and I would maybe like to take her out on a date or something. How would I go about talking to her on facebook (in a private message of course) just to say hi and stuff, what should I say to get the ball rolling to where I can ask her "Hey would you like to go out for a drink"

Like how do I start that conversation off after we haven't talked for so long??

Im a pretty shy person that's why im asking lol