First of all, PRACTICE, PRACTICE, PRACTICE! Then, on the day of, relax as much as you can before the exam, if you practice right before, you could psych yourself out.
Also, I don't think it's about the content of the conversation so much, but if the grammar is correct and your pronouncing things right. Try to think of it not as a conversation but as a project. That's how I suffer though those evil class discussion oral presentations (I've had two so far)
I wish you luck, and be grateful you know another language. I have tried for years and years, and though I'm really good with English (when I want to), French (which is the second language here) doesn't work for me. I'm just not able to think in a second language, it is a weakness of mine that I'll probably pay for later (because being bilingual is super important where I live). Sorry for rambling 