Research scholars generally choose their dissertation topic with their preferences or by following the suggestions of their professors or lecturers. You can pick a suitable topic with the help of your lecturer. Even if you have made up your mind on your own that you have to research a topic, you should discuss it with your professor or any of your senior. They would suggest some improvements, if they would feel like. Before deciding upon a topic, the students should weigh up that is it really a topic that can be researched?
You should never go with the topic that you are not comfortable with. It will be disappointing when you would come to know that the topic, you have chosen, does not have sufficient sources to dig. It is true that internet has answer to every question, but sometimes it gets an uphill task to find the right source on the internet, and you should remember that you have to submit your dissertation on or before the deadline. Therefore, you should try to keep your dissertation topic simple and unique. Unique doesn’t mean an alien topic for your subject; for instance, it would not be wise choose a topic related to national polity, when you are a scholar of physics. Your dissertation topic should be something that you are really interested in. An out-of-interest topic will only trouble you.
Additionally, the topic of the dissertation should be interrelated to your subject. It would increase your chances of scoring high.
To get your dissertation topic approved, you would need to submit a proposal, called dissertation proposal. In this proposal, you have to explain that you want to research this topic, and your purpose of research for this specific topic. The depth of dissertation proposal and its topic will get your research approved.