How do I prepare for my GCSE german speaking exam?

Record yourself running through the speaking exam, and learn the meaning of what you are saying. Practise by talking to somebody or something - be it your family or perhaps a mirror. Continuous practise of your piece will allow you to improve on it continuously and reduce how stressed you will be during the exam itself. Instead of being nervous, you will get yourself into a rhythm and the exam will be made significantly easier, with recall and fluency being vastly better.