My interests evolved from what I was exposed to. During my undergraduate degree, I really had no idea how to narrow it down. I started working in emergency med doing some cognitive research in two labs (more epi than psych), then I went into corrections in the government (addiction research, radicalized offenders), then I went into oncology and memory/sleep and finally cardiology.
It was a whirlwind experience but it helped me understand what research is and what I enjoy versus what I don't. My area of interest became quite obvious after dipping my toes in each lab.