I think you need to be very careful with this. If you're both young, or if this is a new relationship, you could very easily end up pressuring your girlfriend into doing (or saying) something that she doesn't want to. If you make her feel uncomfortable, that's it - game over, you're dumped.
Without knowing what your relationship is like, I can't really give you any suggestions for what to ask your girlfriend. I can, however, advise that you avoid the following if you're young, or if the relationship is new:
- Anything sexual. She'll think you're after one thing from her, and that's definitely not how you want to come across. Not only will you make your girlfriend feel uncomfortable by asking her intimate questions, but you might even get a reply that'll make you feel uncomfortable. Definitely don't ask her if she's a virgin or how many guys she's slept with.
- Anything about previous relationships. Don't ask, because although you're probably wondering, you don't want to know. If she tells you about her ex, you'll start wondering how you compare. Ever seen the movie Closer? That's a great example of what happens in relationships when you start asking questions you don't want to know the answer to.
I know you probably want to find out this kind of stuff about your girlfriend, and that curiosity is understandable.
However, the fact that you're trying to get this information out of her through playing Truth or Dare suggests you're pretty young. It might seem like a quick way of getting to know a new girlfriend, but it's also a very quick way to lose a new girlfriend. You don't want her to think you're coming on too heavy or too fast.
What Should I Do Instead?
Be patient. In a relationship, you find things out about the other person gradually, and that's the beauty of it.
If you really want to play Truth or Dare with your girlfriend, keep intimate questions out of it, unless she asks you any. By all means, take her lead if she wants to go down that path, but don't pressurise her.