In your upper undergraduate years you should really be exploring the field as much as you can. This means taking a diverse range of courses to expose yourself to the various sub-fields, and getting involved in a research project - both to learn more about that sub-field and to learn about the process of doing research yourself. You should also be reading as much as possible. Look for review articles, read Physics Today, browse the Physics Forums Insights, ask your professors for recommendations. When you find something that interests you, read more about it.
During your explorations, you'll probably find that you have a few topics that you keep coming back to, which is great. These are your interests.
You'll also benefit from thinking in terms of your career. What kinds of skills do you want to develop? What will you do if an academic career doesn't work out? How will you translate from a master's degree in X to either a PhD or a job?
Once you've got some answers to all of these, you start looking for programs that match up with what you want. Assess programs not only by their rankings, but by your own criteria. Look at where their graduates are ending up - is that where you see yourself. If you can, visit the campus and talk to current students and professors. It can also be important to factor in things like the cost of living in the city you'll be in, financial support you can expect, university facilities and general environment.