Read the kama sutra. It's full of interesting topics. 😀 Just kidding
The problem is that if you start saying to yourself that you are shy and dont know what to talk about you will become self doubting and introspective and self judging so that you will essentially step on your own tongue. Just forget about it. You will either talk or you won't. but focusing on it will only make it worce. Who the hell knows what to talk about with girls. Maybe she will be wondering the same thing. All you have is who you are and what you've experienced and digested and formulated as what you think about life. But you are not going to be a book to be read from beginning to end. You don't have to rehearse. If you can relax and just be, conversation will happen spontaneously as a process of the flow of associations. It may die and start again elsewhere, but the more uptight and self critical you become the less spontaneous you will be able to be. Go to the prom, relax and have fun. Ask your date about herself and be sure to listen. You don't want to let associations happen in such a way that everything she says becomes a chance to talk about you. Nothing wrong with talking about you, just make room for the other person. Now forget everything I said and have fun.