Please help me. Read my story and tell me what should i do!?

Well im 15 years old boy, theres a girl shes 16 ( 1.5 years older than me). Im taller and bigger than her and im intelligent enough for her, im sure :). Well anyways shes a grade higher than me , shes 10th grade . I would really like to get to know her better and ask her out. Shes a girl who i wouldent just want to let go but i dont really know what to do :/ So please if you got any ideas what to do msg me here or pm. First of all i try to show her that im intrested in her but usually if i look at her and smile she just looks away. When im near her and looking her in the eyes shes just walking tru , ping her eyes quickly over my and then walking on . I do sports so i got a good form. Well if you got any ideas what should i do please , like please help me :/