I am also starting my first year this fall and I have a pretty good idea of what I want to specialize in (IO and finance or behavioral or econometrics). So far I have identified these fields based on my undergraduate exposure to the various fields and my phd program's specialties. For example, the IO, antitrust and regulation, and econometrics courses I took as an undergrad were my favorite courses as I looked forward to each lecture and had passion for learning the subject (unlike in the public choice and economic history courses I took). And as for my phd program's specialties, I am going to UCSB and their program is especially strong in environmental, behavioral, and experimental economics. While I have had no exposure to any of these fields, I have taken the time to learn a little bit about each and found that I do have some interest in behavioral. You may want to look for your fields this way because your school likely has a strong set of faculty dedicated to research in the fields where it is particularly strong, which would make your research opportunities much more vibrant and flexible. Additionally, if your school has a strong reputation in a particular field, it would be less difficult to find a job after grad school that enables you to continue working in your chosen field (and you would have a larger set of jobs to choose from).
Unfortunately though, I believe this choice is binding in terms of finding jobs immediately after graduate school. My father received his phd in economics 20+ years ago and specialized in public economics (along with IO, but his dissertation was in public economics). Because of this choice, his career options at the time were severally limited. He was unable to find an academic job in an environment that appealed to him (Iowa did not appeal to him) and thus he had to leave academics to pursue his career as an economist. He has advised me that I should choose a field where jobs in academics and the private sector are readily available.