A dissertation is a really exciting thing to be working on. Part of the fun of it is brainstorming ideas and possible topics to write on. A good way to go about it like others have mentioned is to consider why the subject attracts you and what you enjoy about it.
You should also talk to your academic advisor and tutors about ideas for your dissertation. They would be a good point of call for any questions you have about the subject and can give you sources and suggestions on what to research.
You should also understand that oftentimes the topic of your dissertation will change and develop as you do more research, so do not feel too worried about not having a clear idea of what you want to write specifically.
Goodluck with your dissertation and I hope you enjoy the process.
Lancaster University Student Ambassador,