The first and most important decision in research is what field to study. Unlike in undergraduate which offer student in broader knowledge, graduate school offer more focus, it will prepare us become expert in very very specific filed. So choosing right field of study definitely will set your future path more easy.
Then if you ask me what is my field of study?
i am a water engineer
especifically in biological water treatment
and why you choose that?…….. My answer is simple
picture taken from here