Practice, practice, practice
The best preparation you can do is to practice speaking French as much as possible. Practice with your friends, record yourself on your phone or even watch French TV, (TV5 for example, or choose International - French films/TV on Netflix!).
The more you do, the easier your conversation will flow on the day of the exam and the more confident you'll feel. If you do little work or practice in the run-up, you're only going to be nervous and be more likely to make mistakes!