Topic Selection
Finding a topic which you are most familiar with is fairly easy, after all, you are drawing upon your own experience and knowledge. even if you don't feel pretty confident about delivering the topic which you thought you know, you can always look for more informations by doing a little research.
If you are invited to speak before a group of people, chances are, you already knew the topic which you are going to speak about. Say you are an enviromental activist, you have been invited to give a talk, wouldn't the organizer know your about it beforehand, would they instead invite you to talk about the latest Mars exploration when you know nothing about it! First thing first, say you are being invited to talk about the environment, well it is a general topic indeed. What are you going to talk about, covering everything from A to Z? It gonna takes more a week to do so... it's a huge topic, and that is a whole academic degree on this topic alone! So what do you do? Break it into subtopics.
Topic: Environment & Us
Subtopic: Protecting the forest
But it is still vague... How do you go round protecting the forest?
Specific Purpose: To inform the audience about the consequences of deforestation.
Main ideas:
a) Disease
b) Loss of species
c) Extinction
Once you have chosen a topic, you must narrow down your choices to something specific as shown above. Say if you are going to talk about US trade deficits, you don't have the whole night to talk about it. So what are you going to do? Narrow down your topic to something precise and specific!
Topic: Natural Disasters
General Purpose: To inform. (Notes: To inform, persuade, or argue?)
Good! But what do you want to inform about? Types of disasters? Steps to responding to disasters? or the role of the Meteorology's department in US?
Specific purpose: To inform my audience of the major step in responding to natural disasters.
Well done! Always cut off any irrelevant points/ideas that you might have included. Keep it precise and specific! Let's try another example, say you would like to talk about science! Oooh!... a huge topic, people spend years to complete their doctorates, it's impossible to summarize everything within 2 hours talk, even if you could, more often than not, it's rather vague. look for something specific instead; say to inform my audience about the benefits of genetic engineering in tracking criminals' DNA profiles? How do you think? Always limit your purpose statement to one distinct idea and never make your specific topic to be either too vague or detailed.
Say you want to talk about the use of computer;
Specific purpose (vague): To inform my audience about computer?
Specific purpose (detailed): To inform my audience about the history, developmnent, assembling of computer parts, configurations of computer system as well as the science behind it. (duh!...)
There are unlimited resources around where you could use to research about the topic; Internet, being the largest library in the world, or your local community libraries, virtual libraries, multicultural resources and on and on.