It's in your benefit to narrow down as quickly as feasible for you. I realize that it's hard as a pre-clinical or even clinical student without much exposure to these fields. Obviously one early branch point is medical specialties vs surgical ones. These are very different from each other with respect to lifestyle, training, and culture. It's definitely one of your earlier decisions to be made.
In terms of competitive fields, in-field research > out-of-field research >> no research. So it's not the end of the world if you take a while to narrow down but the longer it takes, the more you're going to be pulled in different directions (doing research in two fields where half of your research will be less relevant come residency application time). Once you narrow down, you can focus all your energies on that one field.
Obviously this isn't to say that you should make the decision before you are ready. Just think about what factors/information you still need to make a decision and actively try to seek out that information, whether it's talking to residents or more shadowing.