I'm trying to get to know a girl. i need advice.?

i like a girl in my school but she is a grade below me. but we have 3 classes together.although i don't talk to her i find that we have a lot of things in common (i.e) music, attitude, fun, n artisticness. it's 2weeks until the end of school and im starting to get around to talking to her. my goal is to get to know her by the end of the school year, build our friendship over the summer and maybe take things to the next level during that time or when school starts again. the only problem i have is her sister, i went out with her sister's friend for a year and tht relationship turned out crappy.n now we see things in our relationship from different perspectives.n im not sure if my ex told her sister a bunch of crap tht isn't true about me. n if thts the case im not sure her sister will approve. but my problem is is tht i don't know how to talk to her.when i talk to girls i get the feeling tht they know i want to be more thn friends.how should i get around this?and wht could i do?I'm trying to get to know a girl. i need advice.?
you know what she like to to thats a plus rite there just ask her ya wanna check this out but it has to be something she likes and yall both go and see it together but stay the same way as u are around her now be ya self