How can I give a speech when the topic is given to me on the spot? I can't think quickly enough!

Usually you are given a small amount of time to think a little, even if it's only five minutes. During this time, focus on introduction, three points and a conclusion. Keep it simple and draw on everything you possibly know about the topic. If you know nothing about the topic, think laterally and draw on what you do know to illustrate how things might work for the topic you don't know. If you're allowed to google it, then look up an encyclopedic page on the topic to assist you. When giving the speech, stick to the basic structure and don't waffle, as it's best to create a great impression quickly than to belabor things and bore people with an attempt to say too much.