How do you make a christian girl like a bad boy?

It depends very much on how strong she feels about being christian. If she's more like christian because she's always been she most likely doesn't mind if you're not. If she feels strong about it though, try to keep an open mind towards it. Let her tell you why she feels it's good and maybe consider accepting parts of the religion that you like.

As for bad boy good girl... you'll have to talk to her. It's your personality and you shouldn't hide it from her. It might be good if you are aware that what you've done in your past wasn't right (depending on how "bad" you are) and tell her that, maybe even ask her for help to better yourself.