Even if you have a topic you like and that you believe your audience will find interesting, you are more likely to have a successful speaking experience if you do some thinking about the reason you are speaking in the first place.  What is your purpose?

While you may have an idea of your general purpose (to inform, to persuade, to entertain), you will benefit by taking the time to identify your specific purpose.  The specific purpose is a statement of your goal for the speech - exactly what you want the audience to learn, or what you want the audience to accept, by hearing your speech.  This statement may never show up in your speech (or it may develop into the thematic statement), but it is important as a guide for you to follow as you plan your speech.  

Some statements of purpose are more effective than others.  Generally, the effectiveness hinges on how clear and specific the statement (and therefore the goal) are.  Compare the following examples.

Ineffective:  To inform my audience about the Civil War.

More effective:  To inform my audience about the role of spies in the Civil War.

The second statement is more effective because it focuses on a specific aspect of the broad topic of "the Civil War."  The second speech will be much easier to prepare because the speaker will have a clear idea of the type of information he/she will need to gather for the speech.

Ineffective:  To persuade my audience that the federal government should ban all cigarette advertising and that smoking should be prohibited in public buildings.

More effective:  To persuade my audience that the federal government should ban all cigarette advertising.

The first statement really belongs to two different speeches.  In general, watch out for the word "and" in your specific purpose statement.  It may mean you are trying to do too much for one speech.