You have to have a topic for your speech.
Some great advice on picking a topic:
http://www.speech-topics-help.com/
http://sixminutes.dlugan.com/speech-preparation-2-select-topic-idea/
http://www.wikihow.com/Choose-a-Topic-for-an-Informative-Speech
Still not sure? Try this site for some suggestions: http://www.hawaii.edu/mauispeech/html/infotopichelp.html