How do I talk to this girl?

I am 15 and a freshmen in highschool. I have been chatting with this girl A LOT(like hours and hours everyday.) But we have never spoken in person. She is in 8th grade but same age as me. I see her everyday and we look at each other. I have spoken to her when friends are around but never alone. I just want to know how to start a conversation with her? What to talk about? How to approach her? It would help a bunch! Thanks in advance!