A thesis statement states or outlines what you intend to prove in your essay.
A good thesis statement should be clear, concise, specific, and takes a position.
Whether you use a diagram or outline doesn't really matter.
Some people prefer and work better with the flowing structure of a diagram.
It's important that you learn early on how to write effective essays that communicate clearly and accomplish specific objectives.
An essay is a written composition where you express a specific idea and then support it with facts, statements, analysis and explanations.
Others like the rigid and logical structure of an outline.
Don't fret, once you get started, you can always change formats if the format you chose isn't working out for you.
It introduces the reader to the idea that the essay will address.
It is also intended to capture the reader's attention and interest.