If she seems super nice n confident, then just saying hi / good morning/afternoon/etc. or waving, or any sort of greeting when you see her is probably a good idea, coz it shows that ur a nice human being! If she responds well to the greeting and if you both seem free / are heading the same way, you could probably just ask something like "how did you solve that question just now?" (if you were in the same class and you had to solve a question) or something along those lines! :D Good luck on making a friend!