A call to action in a speech refers to the portion in which the speaker informs the audience of exactly what it is he want the audience to do. Possible calls to action include signing a petition, making a donation or spreading the word about a specific cause so that more people can be made aware of it.
A call to action should be delivered with passion and tact in order for the audience not to feel too much pressure and so that it's clear just how important the cause is to the speaker.
Before the call to action, the introduction and body of the speech should describe a cause, organization or individual. The speech might also include what the future plans are for the cause, organization or individual so that the audience has a solid idea of what their time, money or energy is being used to accomplish.
The audience needs to feel as though they are part of the solution whenever they hear a call to action. Not only should the call to action have an impact, the speaker's exit should also be memorable. For example, there should be genuine applause or agreement that indicates that the speaker's message was both received and understood.