How to start and continue a conversation with a guy over Facebook?

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So I want to message this guy I've liked for a while now. I'm in high school and I see him at lunch, but I don't have any classes with him this semester. He's more of a guy who gets hit on by every girl in school, but he's not dating any of them (as far as I know he doesn't) he keeps more to himself, like he doesn't have to be the Center of attention and that's why I like him. But I wanna take it farther and message him over Facebook. If I say 'hi' an he says hi back like maybe I'll say what's up? But like what am I suppose to say if he says nothing, and then the conversation ends and he never messages me again. I'm gonna feel like an idiot. I don't want to message him every day just to keep a convo going and I be the would never message me on his own because I'm still not sure he even likes me . (He's a popular attractive guy and I'm just normal me, not the pretties girl in school that's for sure) any ideas? I'm really nervous and I don't wanna mess up, help ? Thank you!!