If you begin your thesis research prior to defending the proposal, you risk losing all work done to date if the committee requires revisions. The proposal should be defended within a year of writing the comprehensive examination. The topic you choose will be a major determinant of your success in the program and may also shape the direction your career takes after graduation. As well, going through the experience of selecting a suitable topic is excellent preparation for a research career in which you will often be selecting research questions. The selection of your thesis topic is a creative process and, except under unusual circumstances, should be done by the student, not the advisor.
Advisors have a major role to play in helping guide the student toward a good choice.
The process varies from individual to individual but often is as follows. The student first selects the general area of interest and then independently reviews the literature in their chosen area to determine what study would move the state of knowledge forward. Another variant might occur when the student has not as yet decided on the general area but reads something which stimulates a question worthy of inquiry. A good rule of thumb is that, when completed, the study should be suitable for publication in a reputable Epidemiology, Biostatistics or Public Health journal. However, the scope of a Ph.D. dissertation should have more depth and dimension than a journal paper study.