How do I get better at impromptu speeches for my speech class. I'm terrible.?

To be totally honest, that is the most ridiculous prompt I have EVER heard. Aside from simple practice (maybe have your friend or mom come up with prompts on the spot), I would suggest faking it. For ridiculous questions like that, fight fire with fire. Come up with the most ridiculous reasons possible, and act confident, like you really believe in them. Smile, be full of energy. For serious prompts, you don't necessarily have to argue your side. Think of the first serious, juicy point you can and go with it. Good luck!