A graduate research dissertation is not only a more comprehensive document than most students have previously prepared, but it also requires a particular style of thinking.
Central to a dissertation is the thesis: the core idea or argument. In the words of Anderson and Poole (1994):
“a thesis is not the same as a topic to be investigated… while a topic may be the structure of a particular crystal or the novels of Miles Franklin, a thesis is a statement that says, for instance, something about crystals or an author’s novels. The distinction is important since in many ways the statement of a thesis determines the approach and stance writers take towards their topics or research questions… a thesis is an idea or theory that is expressed as a statement, a contention for which evidence is gathered and discussed logically.”
Thinking about the thesis should start at the very beginning of the project. It's a good idea to read about writing dissertations.