I really want to ask this girl out but I don't know how...HELP!!!?

I'm 15 and I have never had a girlfriend before but there is this amazing girl that I met 4 months ago and I really like her. We met through track at my high school and we really started talking about three months ago and then suddenly around my birthday (2 months ago) she started getting really flirty and I loved it, since then we talk and text non-stop everyday. We have talked about being a relationship kinda flirting but kinda serious but she has told me that she likes me and she says that she misses me all the time. She's even said that she wants to kiss me and things of that sort even though we aren't officially dating. I really want to make a move on her and date her because we really click and she has been one of my best friends since I met her but I am very inexperienced since I have never had a girlfriend. I am worried that if our relationship doesn't work out that we won't be friends anymore. And we are going on a "date" to the movies on Friday and I am spending the day at her house on Saturday. I really think that I am in love with this girl. Any advice?