First, know that writing a dissertation is a long process and should be well thought out from the beginning. Ask yourself a few simple questions: why did I start my study at the graduate level? What am I the most interested in? Your answers to these questions should be major clues for pinpointing your dissertation topic. After you have a topic then you need to narrow it down to something you can, within reason, research and write about in 200 pages. This you need to discuss with your adviser. After he or she approves your topic you are on your way to starting your research.
Read other dissertations on a similar topic so you can find out good resources. Do research on your own too. This will help in your literature search.
Of course you need to do an experiment or some kind of research. This should be thoroughly done with quantifiable data. It is good to get help from a statistician to make sure your data is analyzed properly.
Then the discussion section will follow the methodology section where you write up your research.
Finally the conclusion section with suggestions for further research.
I will write many articles on this topic and post them here on hubpages.