Ask a girl out if you believe you can have fun with her. You should feel relaxed with the girl you are asking out, though it is not totally mandatory that the first date must be super romantic in a Hollywood sense. It can just be a friendly date with no strings attached. If you are terribly infatuated with the person, you'll find out whether the girl is also interested in you. Try and date someone closer to your age when going on your first date.
It might be a great idea to portray yourself sincerely without faking it or trying hard to be someone else. Try to not show off and never come on too strong with the girl on your first date. Teenage dates are something more transient than mature or adult relations, so you can be relaxed and untroubled without worrying too much about the relationship. You can try out a first kiss on your first date, but do not fret about whether it is clumsy or not - it is most probably going to be ungainly anyhow. Because you are still a teenagers, you could feel awkward with a girl at the start and start wonder about her feelings.
Always offer to pay for the snacks or any other movie tickets and so on. Your date will love and respect you for it. But if she swears, you can go Dutch with her paying up for a 1/2 the spending, but this should be done only after you've initially offered to pay for the whole outing. Be kind and conscientious to your girlfriend and do not just treat her like a sex object.
Try to listen to her and give her respect. Try and be chivalrous, which is terribly rare these days. You can open doors, carry books for her, present her with flowers and be deferential to her parents while you greet them. Your first date is highly important to you, so keep this advice in mind, so that you will have the best first date ever.