Your question is a good one as there are over 4 dozens specialties in the discipline of psychology. Learning what you are good at doing AND what really excites you (curious, passionate) are the two, key ingredients for honing in on a psychology specialty.
I pursued a doctorate in quantitative methods in psychology, with an emphasis on assessment. I was always fascinated by the predictive ability of many psychological tests (especially vocational interest inventories which I took as a high school student; it recommended that I pursue either psychology or mathematics). When I learned more about the field of psychology as an undergraduate and how important mathematics was to the creation of assessment devices I decided to go into this specialty. I was always very good at mathematics and loved it...so I combined both passions in my doctorate.
I specialized in "personnel psychology" as an I/O psychologist for the better part of my career - 23 years. I discovered later in my career that I loved teaching, as I was doing a lot of training and 1-on-1 coaching. I was blessed to find a full-time faculty position at a small, private university at the age of 53 and taught full time for 10 years. I am now retired and do a little bit of human resources consulting and continue to teach part-time - one course per semester - at a community college.