This is your first time with her, so you will inevitably be nervous. I suggest that you keep a few fruit plates and a bottle of red wine with you when you are making it, and eat while you are making it. Also, before you do it, you have to learn to flirt, let her relax, and fully enjoy the joy of sex. Also, you have to be gentle and don't act like a reckless man. Your girlfriend will be very painful for the first time. Try to ejaculate outside her body. After all, she Can't get pregnant yet. Finally, tell your brother that your first ejaculation will be very fast. This is normal. Don’t think too much about it. Treat her well. I wish you a happy life.