Depends on who asked who out. If the girl asked the guy out, she should offer to pay for both of them (most of the time the guy won't let her do that, but it still shows some class on her part) and vice versa.
Now, if you don't really like this guy you are on a date with, you need to pay for your own meal. Most of the time this will tell the guy that you aren't interested without having to say anything.
But it also kind of depends on the couple as well, like where they are financially, say if you date a guy who is flat broke, don't expect him to pay type of deal........(oh and watch out for those guys some of them can be sneaky)