Research areas are always evolving, especially in computing: what might be a hot topic now might not be actively researched in a decade. And there are many considerations around choosing a research focus, such as what interests you might already have developed, what areas might have sufficient funding to support research, and what problems in the world might deserve your attention. As an undergraduate, it might also be important to just focus on the research areas with opportunities at your institution: a department rarely has expertise in every area of research, and so local options might be constrained.
If you’re not sure what your interests are, one strategy can be browsing recent research and seeing what topics seem interesting. An easy way to do this is scan the conferences and journals in the ACM digital library, which have the titles and abstracts of all recent research available for free. Are there questions or inventions that seem particularly interesting? Who are the researchers investigating them? Are any of them at your institution? Research can be a small world, so you might even consider writing an email to the researchers to ask them questions about their current unpublished research.