How do I show this girl that I like her?

I'm 15 and a sopre in high school. I recently broke up with my first girlfriend, and was devasted. So, in an attempt to get my mind off of her, I'm trying to put myself back into the dating world. So, I recently found a girl who I like. I recently realized that I have kinda liked her for a long time, but never noticed it. We are casual friends. I have known her for many years, because we have had identical schedules since 7th grade. I talk to her constantly, because she's in all of my cles. We've never hung out outside of school, but when we talk we usually laugh a lot. We bicker sometimes, but it's always good natured. I playfully tease her a lot, and she jokingly slaps me. She recently broke up with her first boyfriend, and I figure this is my best chance. based on the nature of our friendship (I don't know if you can call it a friendship...we aren't that close), I don't think she would take me seriously if I ever made a move. Recently in biology cl, we had to chose lab partners and she asked me to be hers. Of course I said yes, and then I asked her for her phone number in case of homework. So I have her cell number, and have texted her only once regarding homework. Never anything else though. I often make comments about how hot her sister is, but that is mostly to annoy her. How do I show her I like her, without being creepy? And how do I get her to take me seriously?