Your thesis statement should clearly articulate the purpose and main points of your speech. Think of the thesis as the rocket that will guide the spaceship, that is your speech. It's there at the beginning and, in some ways, it guides the trajectory of your speech.
Defining a thesis is essentially constructing the structural outline of your speech. When you have defined a thesis, you have essentially articulated to yourself what your speech is going to say, what position you will take up, as well as what is the speech's purpose.
Use the work that you have done to narrow down the scope of the topic that your speech is about; determine the purpose your speech will serve, and define a thesis to construct the remainder of it.
Begin looking very generally at your speech:
From there, begin to refine and hone your thesis by getting more and more specific, until you are able to define anywhere from one to five main points that you seek to make with your speech. It is typically only one to three sentences long.
The thesis should be introduced near the beginning of your speech, usually at the conclusion of the introductory remarks. Its placement there is a way of introducing the audience to your specific topic. It should be a declarative statement, stating what position you will argue.
It's also particularly helpful to give a quick outline of just how you plan to achieve those goals in another few sentences, immediately following your thesis statement.
At the end of the speech, you should restate your thesis (perhaps in a more concise form) in order to reassert to your audience what you have argued throughout the course of your speech.
Source: Source: Boundless. "Defining the Thesis." Boundless Communications Boundless, 3 Mar. 2017. Retrieved 29 Jun. 2017 from https://www.boundless.com/communications/textbooks/boundless-communications-textbook/choosing-a-topic-6/establishing-a-purpose-and-a-thesis-36/defining-the-thesis-160-1434/