There are two things you should consider when asking a girl to be your girlfriend. First you want to be sure that you have really gotten to know one another. You want to know for sure that you are interested in one another and that you want to have a relationship. If that's the case when you ask her to be your girlfriend, it will have much more meaning to her. Asking her later to be your girlfriend is better than earlier. It's better ,if she looks forward to you asking her to be your girlfriend. You don't want her to be surprised and think that it's way to early. Second how you ask is also very important. You might not want to ask directly because it can be very awkward. An interesting, creative, and romantic way to ask is to introduce her casually as your girlfriend to a friend or some casual acquaintance. If she has been looking forward to being your girlfriend she will be thrilled if not, she will let you know later.