I texted a girl that I have crush on to have a study break. We are going to get yogurt in two days but I didn't say it was a date. How do I showed that I am interested? Should I pay for her food?

Since finals are coming up and it's followed by winter break/end of school year, how would I proceed to ask her out assuming that things go well (I really hope so!)?

I've had lunch with her once before but I didn't know how to make a move, how do I tell if she only considers me as a friend?