Just simple, casual conversation. Try not to force yourself into saying anything just to keep a conversation going. I know it's cliche to say, but keep away from the 'superficial' topics like school, weather or work. Maybe start off by asking questions like how family is, or life in general. I usually ask a lot of questions to keep the other person talking, since personally I am much more of a listener than a talker. However; make sure not to make everything you say a question. Maybe if you ask about their family, keep bouncing ideas off, like maybe if she mentions something about her mom, relate it to your mom or whatever.
Make sense? Haha this was kind of spontaneous. But she's your friend and wants to hang out with you, I'm sure the last thing she is doing is judging you so be yourself and just relax :>