Well, I'm not sure if I'm too late to answer this question or not (you said a few months, so...), but I'm going to try to do so anyways. When I was confirmed, I didn't have an Examination, so I can't tell you what to do from experience, but I think that if you have paid attention in classes and know the basics and fundamentals of what you are learning and what responsibilities you will have once you are confirmed, you will do fine. Just be confident that God will be there with you, helping you do your best.
As for tripping in front of people/generally embarrassing yourself, again, just be confident that God is with you and that your family is there to support you on your faith journey. They're there because they love you and want to see you grow up into a lovely "young, Christian woman". Be happy that you have so many loving people in your life who care about you as well as God; that will give you self-assurance. That self-assurance will also help you to walk in your gown (if it really is 20 feet long, haha). People who are confident and self-assured walk gracefully and stand tall, with a posture that says "I'm proud of myself, who I am, and who I will be", and when you have that posture and that attitude, it's very unlikely that you will trip. :)

I hope I'm not replying too late and that this helps!