The topics that you choose for your speech are based on the type of speech that you are making. In speech writing as in essay writing, there are specific purposes that control the choice of topics.
In speech writing, your basic speech purposes include these four types:
Informative-to convey information
topics might include: types of fireworks, history of the modern Olympics
topics: Why I get my headaches? The dog ate my homework.
Persuasive-to inform and convince
topics: Fireworks should be banned; No drivers' licenses before the age of 18
Commemorative-to give honor or esteem
topics: Introducing a famous person; Giving a biography of someone of importance
You must know what your purpose is in giving the speech before you can make the right choice for your speech.