The question is a contridiction in itself. Impromptu speeches are exactly that, you shouldn't or don't have time to have a topic in mind. It generally stems from someone asking you a question or asking you to give a presentation minutes before you are scheduled to speak. You should prepare just as you would for any other speech. Gather your thoughts together in an outline; intro, body, and conclusion. You should be as prepared as possible but it is impossible to be fully prepared because it is impromptu.