1.Review what you are asked to speak about.
2.Think about what you are asked to speak about. Use keywords to help to get material on what you want to speak on from your mind.
3.Be calm and breathe freely.
4.Don't think that you may make fool of yourself. there is no way you are going to make a fool of yourself. It takes a lot of courage to speak in public about anything especially without preparing for it earlier.
5.Try to remember anything you can which has relevance to the topic. Any thing from news articles, scientific journals, or even personal experiences.
6.Speak freely and smoothly, and speak as if to a friend to whom you have spoken for a long time.
7.Try to keep your speech an appropriate length of at least 2 minutes.