Select a general research area that interests you. Develop your topic from a point of interest in the larger field you chose to study.
Read widely in your general area of interest. Choosing a Ph.D. topic is about finding a problem with the existing scholarship such as answering a question that is yet to be answered. Obtain a reading list from your graduate school department if one is required for comprehensive exam purposes.
Look for problems within existing scholarship. Look at the work of other scholars and search for poorly drawn conclusions that have been established based on poor evidence or that do not logically follow from the evidence presented. If your reading reveals that new evidence may now contradict the work of existing scholarship, this can be an area from which you can develop your topic.
Narrow down your research to one specific problem in the existing scholarship or one particular question that you believe needs to be answered. Research the topic to ensure that you will have enough data and resources to write a dissertation proposal and the dissertation. Dissertations are typically book-length projects, so you will need to be able to provide substantial evidence in support of your main thesis.
Modify your topic to meet the parameters required by your graduate school department. You may have one topic in mind, but your dissertation committee may make recommendations based on your dissertation proposal. You may find that your committee thinks your topic is too broad or too narrow or that the research you're proposing is simply impossible to complete. Use their suggestions to your advantage.