Dissertation starts with a topic. And topic appears after thoughtful reading, searching and consulting with professionals. If a topic is really interesting, it is much easier to write a dissertation, at least because of the fact that you feel interested in writing one. A topic you chose should be discussed with the supervisor. He/she can first of all tell you how successful or failing your choice of a problem can be, and secondly guide you to some helpful sources of information.
 
When thinking on how to write a dissertation, think about time management. Dissertation is long term writing, and you will have to give up something in order to succeed. To write a dissertation you also need to feel aspiration, the desire to research, read, note, understand and dig for more.
 
As to dissertation structure, concentrate on general requirements and leave the rest to time, devotion and consultations. You simply need to know basic characteristics on how to write a dissertation. The Abstract has to be short and informative enough to give a clear picture of content, sources and your results. The Introduction should be motivating enough to write a dissertation. It has to have important and absolutely critical things: the thesis statement, hypothesis and the research question. To write a good dissertation you will also need to learn to make scholarly predictions and be very selective.
 
Another answer on how to write a dissertation is to do a good job on Literature Review section, which can be organized thematically, and on Methodology (including your summary of methods and types of researches used). Finally, know what you achieved while working. This is something you need to write in dissertation conclusion.