That depends on how the curricula at the two universities are structured and the specific courses you've taken so far. In the US, at many schools, in your junior (third) year, you would have sufficient background in foundational courses to start taking specialized electives (first or second semester) such as introduction to solid-state physics, introduction to nuclear physics, introduction to astrophysics, .... So pick an elective appropriate to your chosen research field. In my instance, I knew that I wanted to pursue solid-state physics for my PhD, so I chose introduction to solid-state physics and related courses. I later asked the professor of the solid-state physics course whether he would serve as advisor for my senior thesis. He agreed. We discussed several options, and I picked one.
If your curricula doesn't accommodate such an approach, then talk to professors working in the research area that you're interested in concerning potential topics for your undergrad thesis. When you pick one, ask your future advisor for guidance on how to proceed under your university system.