djams,
You go through API first, about 6 weeks, which is just academics. (engines, weather, aerodynamics, etc.) You then go to a primary squadron for the T-34 flight training. After the syllabus there (6-8 months when I went through) you are told what you will fly. (Props, Jets, or Helos). It is based on your grades and the always tormenting "needs of the Navy". Jets are the hardest to get, then E-2/C-2, then props, then helos. So in reality, if you want to fly a helo, it isn't that hard to get them. The needs of the Navy might bite you, but usually not.