How Do I Talk to Her?

I'm a Junior in high school, and I've never had a girlfriend. I've always over complicated things or tried too hard or some sort of circumstance has always stopped me from getting the girl I like. I'm getting kind of frustrated this year and I'm trying more than ever to find the right girl. I'm interested in this girl right now who I don't have any classes with. She came to my school halfway through Sophomore year (I go to a private school) with her twin brother, and they sat behind me in religion class, so I got to know her a little bit then. This year, I only see her around school and I have one class with her brother. Our school has Chromebooks now, so yesterday I messaged her on Google's chat app and asked for her number. I texted her so she'd have my number, but she didn't respond, so I'm worried already she doesn't like me, although she's always friendly to me when we talk. I know not to over text her, but I know I need to talk to her so I can get to know her. I have no idea what to say to her though besides school stuff and to ask her about dance because she's on a couple of dance teams. How do I talk to her, show my interest in her, and make her interested in me? If I can establish a good relationship with her, when is it okay to ask her on a date? Other things? Thanks for your help!