Just keep in mind that the examiners (who will listen to you audio) are not checking your knowledge about the topic, they only care about your english speaking skills.
I remember the topic I was given and how I didn't know much about it but I got a 1 'cause whatever I said, even if wasn't related, I was fluent and confident while saying it
To practice, pick a topic, start speaking on it and record your speech or whatever and then listen to it once you are done.
This way you will know how you sound, are you fluent etc.
Check this for some topics:
http://www.speech-topics-help.com/oral-speech-topics.html