How do I "woo" this girl? please help?

Well i really like this girl she's my best friends little sister, she's a year younger than me and shes beautiful. We're both in secondary school and at the end of the day we both wait at the bus stop for two different busses with some of our friends. We were all chatting and her an I were messing with each other, she was playfully pinching me and laughing, another day we were flirting and she winked at me. The last time I say her was at lunch time (she didn'ttake the bus that day) she was walking by and i started checking her out she started looking at the ground as she was walking and every so often she kept looking up and smiling and sort of fixing her her (brushing her fringe from her forehead with her hand). I was just wondering how can I really impress her and take the final step by asking her oit?