Not sure how to pick a dissertation topic? Read. Identify your interest area and read as much as you an in that area. Take notes on interesting studies , theories, and ideas. Brainstorm your own ideas. Not just one idea. Many. One of these ideas may become the basis for your dissertation.
Look at what fellow students turn out. This will help you learn about the scope of the project and general expectations. Who knows? You may learn something and get some ideas.
Identify Potential Dissertation Committee Members
Consider faculty who you might ask to serve on your committee. You may know many faculty from class and in the hall, but do you really know their work? Read what they've written to determine how well they fit your interests. Identify faculty for specific things they contribute, methodology, theory, etc.
Plan Data Collection
No, you can't collect your data (especially if you don't know your topic!), but you can think about sources of data. If you're working in a social science field you likely will be collecting data from people. You may not know your exact research question but you might know that you'll need to interview or survey specific groups of people, like college students, health care workers, or parents of preschoolers, for example. Think about the sources of data available to you - and if you hope to conduct research with a population for which you don't have access, think about how you'll get access, work around the problem, or possibly change your research topic.