You say something that they would want you to say. I'm a girl so it would go something along these lines... your really cute, smart, and nice. I would always love you and that would never change. Something like that.
tell her how you feel if you don't have anything to say just say "I don't" (; truth is if you have to second guess or hesitate or think about why you like/love the person your with, or why your with them, then there is something in the relationship that is missing... you should know why you love you girlfriend or boyfriend, you shouldn't have to go online to find out why you like/love someone and if its just a crush on someone and they're asking why you want to go out with them then you should already know the answer you can like someone and not know why you like then.. it may be hard to explain but just say what comes to mind