Your dissertation is probably going to be the most important piece of writing you will ever complete, so it's essential that you choose the right title and/or topic to write about. Especially if you're looking for a long-term career in academia, this will become the piece of writing where you will build the foundation of your professional reputation.
There are many different factors to think about when choosing a dissertation project, but your previous years of study should have provided you with some ideas about the area you want to research, and given you access to basic theories and academic writings from which to base your preparation.
Choosing a topic that you're passionate about can make all the difference when choosing your dissertation title, but you need to research how it would work as the basis for a dissertation topic. Whilst some theoretical dissertations can rely on pre-existing literature and research findings, the chances are you will have to factor in conducting your own research, which will undoubtedly take time and possibly money.
If possible, give yourself a bit more time by reading articles, papers and books in the summer before your third year in the area that interests you. This time should also give you an idea of whether your dissertation idea is simply too big to complete in two academic terms, researching and writing your dissertation to the highest possible standard.
You'll be able to find inspiration for your dissertation topic from your lecture notes, text books, module handbooks, seminar discussions, theories, academics you've read and current affairs.