A few tips to hold back your orgasm when fucking.
1. use a condom
2. drink alcohol, but not too much.
3. have an orgasm before having sex (jack off, let her jack you off, bj, whatever) You can take a 'bathroom break' if you think that would be the best solution.
4. practice now by jacking off, and holding your orgasm. Don't do those last strokes.
5. have sex with an ugly girl
Or...
Have sex with someone you don't care about. One of the main reasons for cumming prematurely is stress and anxiety, which you are going to have when you are with a girl you want to show a great time.
But the best thing according to me is, tell the girl just before it's about to happen. I mean at the point (you are naked and) you are sure it is going to happen. Tell her you are a virgin. Apparently girls like taking someones virginity, and since they know you are a virgin, they are going to be very understanding and take charge. This way it's going to be a piece of cake. You don't have to worry about what to do, she will tell you. And mistakes will be forgiven.
When I lost my virginity, the girl I was with thought I was experienced, I never corrected her. Of course it was a fiasco. I told the second girl I was with I had only done it two times. She probably suspected it was going to be very bad. And it wasn't the best she ever had, but apparently not the worst either. She gave me confidence and told me I was a fast learner. We did it a lot the next couple of weeks, and although she wasn't my first, I consider her my real first.