Help me talk to a girl I have no classes with

So there's this girl in school that I have no class with. I wanna get to know her but I don't know how (i aint got the answers sway). I followed her on insta and she liked a pic i posted, and she often likes pictures that I like (that I'm sure No One else liked). Whenever I see her in the hallways she looks down at the floor and then looks up as we pass. I don't know why her but I really wanna get to know her.

I'm not scared of talking to her or DMing her, I just don't know what to say. Anyone got any tips?