How do I take interest in a girl?

How do I take interest in a girl? I really want to get to know this girl that I'm friends with. We don't really know all too much about eachother, we just chat at school, but the other day we went on a 'double date' (if you may) with our best friends and we both really enjoyed it. I think she really felt comfortable around the me that she hasn't seen before, the outside of school- more chill me. ANYWAY, the boring details are what make this subreddit cringe. How do I take an interest in this girl without it seeming like a joke, or without it seeming like I'm coming at her too strong/ obsessive? At this point I'm not "oooOoohhHO in loooovee", I just want to get to know her more.