I the 5/6 years of medical school is the time to explore all options and develop your interests. What you might be interested in now could be completely different from what you want to do by the time you graduate. Having a rough idea of what you intend to specialise in from the beginning can be a good head start in terms of finding opportunities and keeping motivated, but its not necessary or common.
I'm lucky in that i went into med school already knowing that surgery is the speciality for me, it's hands on practical, difficult, suits my personality and lifestyle, immediate in terms of treating patients and just suits my overall clinical interests. Clinical medicine just doesn't do it for me.
I'm leaning towards neurosurgery (vascular particularly), mainly because neuroanatomy is what I enjoy learning about the most, and if you're going to dedicate 10+ of your life training in a particular surgical field, you best be interested in staring at that body part for hours on end.
However, more recently i'v been getting more and more interested in cardiothoracics, as I'v found a lot of the operations performed to be very complex and engaging in a way that I havent found with neuro yet. The only fear I have with cardiothoracic surgery is the apparent diminishing nature of the field itself. The ratio of jobs:applicants has been steadily growing worse, a lot of the work cardio surgeons used to perform have become redundant as catheter-based treatments for heart diseases become more advanced and cardiologists now perform a lot of the work cardiosurgeons used to do. Hence, It's seeming more and more like if you're interested in specializing in cardio, cardiology is a safer job with a more reliable future than cardiothoracic surgery. So I might just have to stick with neurosurgery