swimsoftball, The answer to your question does change a LOT depending on your service assignment. Here's an attempt at a brief answer.
-Minimum service requirement is 5 years, but the requirement increases with any specialty training. That's why aviators end up with ~8 yr requirement
- Service areas: Most people end up in one of the following categories: naval aviator, naval flight officer, marine corps (aviation or ground), surface warfare, submarines. There are a very limited number (currently no more than 25) slots to attend medical school.
- Do you get choices? Sort of. Everyone gets to put in an ordered list of their top choices. Most people get one of their top two choices, but not necessarily their top choice. Generally, the closer to the top of your class you are, the more likely you are to get your first choice, but some top academic performers get "encouraged" to list subs as one of their top choices. Those who go surface warfare get to choose (in order of class rank, called Overall Order of Merit, or just OOM) their first home port/ship assignment. When in doubt, the needs of the Navy come first.