が is a way of introducing new information to others that don't know. For instance, Bill knows that Robert went to Okinawa, but Sally doesn't. Bill would say to Sally:
ロバートさんがおきなわに行きました。(Robert went to Okinawa).
Also, the particle が indicates that there is information missing about a certain noun.
Ex: だれがおきなわに行きましたか。(Who went to Okinawa?)