How do I become better friends with someone?

I have a class with this girl and she's really nice to me, we talk a lot in class and we wave at each other in the hallway, I would consider us friends. The issue is that I don't have any other friends and I want to be better friends with her but i can't figure out how. Ideally I would like to become good enough friends with her to hang out with her outside of the one class we have together. Also ive never made any friends before, any advice would be greatly appreciated.