Approaching a guy: (how) should I do it?

There is this guy I see every week and I would like to get to know him. He is a bit older than I am though, I am 16 and he is 20. I haven't really spoken to him yet, just greeting each other and smiling and I'm friends with him on social media. How should I approach him? We are never alone so that is why it is harder. What should I say to him? Also, how do I know he might be interested in getting to know me? Because I do see him looking at me sometimes but obviously this doesn't have to mean anything and could be cuz I'm looking at him unconsiously. Any advice for me? Or should I maybe post a reaction on one of his pics or send him a message? Thanks!
Approaching a guy: (how) should I do it?
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