Keep it simple and choose an activity that will allow you two to see each other and talk, like getting something to eat/drink, museum, etc. Just don't choose a movie. Sitting in a dark room for two hours without being able to talk to each other is terrible when first starting to hangout.
Just keep it casual and take initiative, but keep it slow at first. It's really not that hard.
"You have nothing. Nothing to threaten me with. Nothing to do...with all your strength."
This is one of, if not the best single line of dialogue that encapsulates Batman and The Joker. There is nothing Batman can do to frighten the Joker. He is not only unafraid of Batman's brutal strength, genius intelligence and expert fighting skills, he actively welcomes it. He wants to drive Batman to unleash his power and, ultimately, break his own rules.