Before focusing on a specific topic, you'll need to narrow down your ideas to one subject area. Gather information that has to do with your overall subject; you'll begin to narrow down topics and eventually choose the one topic that you'll focus your dissertation on. As you explore your subject, you'll notice trends and ideas that continue to pop up, and you'll also get a good grasp on how much information is out there pertaining to specific topics.
It's generally required that your dissertation be original. This can prove tricky, because so many topics have already been researched and proven. Focus on topics that spark controversy and discussion; this will lead to an area that has not yet been decided upon. Your dissertation will need to be completed within two years. If you give yourself a longer time frame, you run the risk of choosing a topic that someone else will solve within the time allotted, and your idea will no longer be an original contribution.
This may seem obvious, but your dissertation should be interesting. Both you and your professor should be interested in the topic that you're writing your dissertation on. You need to find it interesting because you'll be spending a lot of energy researching and writing on this one specific topic, and it's important that you feel driven to actually do the work. Similarly, your professor will need to read the entire dissertation, and you'll want to hold their attention. You should both be enthusiastic about the topic.
Research information that has to do with another area of the world. You can either focus on a current issue or backtrack and delve into a historical topic that has not yet been researched thoroughly. Family dynamics are a constantly changing topic, so there should be consistently new ways to explore the topic. The same goes for technology, which is constantly growing, changing and expanding.