The best way is to get involved in research as early as you possibly can. Since you are about to enter college this fall, as soon as you can, get involved with whatever project that seems most interesting to you. It may not be directly related to what you will eventually research in your graduate studies, but by getting involved in undergraduate research earlier, you will know what you may be interested in, as well as what you may not be so interested in. You may have to jump around a few times to find out what you are interested in, and you'll also learn the great and not so great things about academic research in the process as well.
It is somewhat less common for freshmen to get involved in research, but before you know it you'll be a sophomore, junior, senior then graduating college. So make sure to get involved in your classes first (by asking questions and going to office hours, or OH), this way you'll get to know your professors. If you find yourself interested in their research, read up on their papers, then go to OH and ask about what their research is, as well if you could possibly assist with it in some way. (From my experience, finding out what you are most interested in, like many things, take some, if not a lot, of trial and error)
Also, while at it, also get started on crafting a statement of purpose or essay about your research interests. While graduate applications are a (seemingly) long way down the road, since you haven't even entered college yet, many summer REU or research internship programs (great opportunity to gain research experience especially if there aren't many such opportunities at your university) will want some kind of essay component as part of the application process.