What should i do about this girl.?

Hi, I'm really really confused. I met this girl the other day at a bbq and I isntantly really liked her. We spent just about all day together until she had to go home. I think I noticed some things that made me think she liked me a bit, such as I caught her looking at me and when I smiled she smiled from ear to ear and she laughed at about everything I said. I think I noticed some dirty flirting aswell, I dropped the can opener into my can and couldn't get it out so I poured what was left into a beaker, when I sat next to me she smiled at me and said "its tiny", seemingly talking about my glass lol. I kept the joke going by saying "you havnt seen it". The real downer of the conversation was the way she made it clear that she had a boyfriend for apsolutly no reason. I just don't get her atall. What I'm wondering is wat should I do next time I see her what should I do? Should I flirt with her, and go out of my way to comliment her and smile at her or should I just talk to her and try and have a good convo. I want to do it right and not ruin it like I normally do with girls coz iv never liked someone as much as I like her. Btw I'm 15 and she's 14 . Thanks for the help