As an ex-Navy physician, here is my advice.
1) think hard about joining...it is not the adventure that you see on tv shows like JAG.
2) If you join afterwards, you are in the driver's seat about where you go, or they just send you anywhere, and as a new guy (read no- rank) , you are the last to choose.
3) Negotiate for everything...loan payoff, duty location, etc. Get it in writing, and even then, they can screw you, because of little clauses they put in the contract. Have a lawyer look it over, if the Navy is not willing to remove certain clauses that let them back out of a deal, then you know what they plan on.
4) If you must join, sign up for only ONE year for a certain duty location....anymore than that, they can send you someplace AWEFUL after the first year.
5) Always be in the driver's seat, meaning you have no obligation to stay any longer than you wish.
good luck