Selecting the right PhD topic is an important endeavor. Of utmost importance is that you choose something you are already interested in, passionate about, knowledgeable with and can see yourself completely absorbed in--because you will be. Equally as important is the possibility of your contributing something unique to an existing body of knowledge, or something completely new and novel. This requires a bit of homework.
Write down your professional interests, what you're most passionate about and list them in order of priority. Consider what research you conducted in the past or what you've always wanted to research. Also ruminate on any special projects you worked on or managed, such as studies or service on professional association boards.
Select one or two of your professional interests and conduct searches in a university library, preferably one that subscribes to EBSCOhost or ProQuest. Conduct a separate search for each topic: a general and a specific one for peer-reviewed publications only. Look for other similar studies or publications on the subject. Peruse the research to ensure that you might contribute is workable and will add to the existing body of research.
Pin down your ideas to one or two, and base them on interest as well as the results from your library search. Consider the number of studies already conducted in the area or in a related area, and what the results were. Doctoral ideas must have theoretical grounding yet be original enough to add to existing knowledge. If it is determined that your work would only duplicate other work, move on to the next topic and weigh those results in the same way. Consider the number of peer-reviewed articles and studies, what was studied and how you can add to that existing body of knowledge.
Select two topic ideas that seemingly allow room for new research and that you will not be duplicating. Present these ideas to your dissertation mentor (if you have one) and with your committee, or discuss it with a professor or advisor from a college or university you are interested in attending. They can give you good insight on the merits of the topic by letting you know of their own knowledge and interest in it, as well as if other students have worked on similar ideas in the past.
Select the best idea for you, weighing in your research on the topic and input from a mentor, committee members and your own passion for the topic. And then prepare to throw yourself into it.