how to select the public speaking topic

You should choose a subject that suits you and also fits into the time slot allotted to you. In public speaking you have a responsibility to other speakers. You will not cut into others' timings and force them to hurry. They too may have something worthwhile to say.

You will talk your best on a subject that you know well or in which you are deeply interested. Remember, when you are talking you are an authority. You ought to know more about the subject than any member of the audience. This should be true, if your speech spins around your personal viewpoints and experiences. Hence, you can consider the following as possible subjects:

Your job or hobby or other interests like games, books, philosophy, travel or out-door activities. The wondeful places you have visited or the interesting people you had seen and interacted with. Your past experiences, beliefs and convictions Some field-of-knowledge in which you are an authority. Your own ideas or a process, the details of which, you would like to share with others. Some projects that interests you. Something you have read, which you would like to discuss. You would like to narrate something, which you have seen and experienced personally. Something good or bad that happened in your neighborhood or county or state or the country at large, on which you want to give your observations. Your conviction or suggestion for solving a social problem or streamlining an administration.

..Or simply, your desires, hopes and ambitions.