I feel pretty qualified to answer this. I am a current active duty Sailor stationed in San Diego on a Destroyer.
I joined after getting offered an interesting rate(job) in the electronic warfare field and decided to withdraw my application to my state colleges in Florida to pursue the Navy. I dreamed about the Academy, but with my 3.1 GPA and no AP classes, it was little more than a dream.
After completing a year of school, I deployed and started my application after my Captain said I belonged as a midshipmen. Here is the basic breakdown for you.
1) You need to graduate basic training (boot camp, 9 weeks long). It was not tough for me, but I trained for SWCC and spec ops for a year so the physical punishments everyday were cake for me. It's pretty much plebe summer with a more compressed training plan.
2) You will have follow on "A" school and possibly "C" school depending on your rate. DO NOT GO UNDESIGNATED SEAMAN/AIRMEN. I can't stress this enough. It will allow you to leave for bootcamp faster (normal time in DEP or delayed entry program is around 6 months from the time you sign your contract, until you leave) but will put you at a severe disadvantage at separating yourself from your peers. I have observed ONE undesignated person actually get a recommendation to put in a package. Choose a rate that interests you since if you do not get accepted you are stuck with it for 4/6 years.
3) After 2-14 months of school (you can not apply to the Academy during school unless your a Nuke) you will transfer to a ship/shore/sub command. You basically need to be a super sailor from the moment you check in with your dress blues, until you depart to Annapolis. Take the dirty jobs that no one wants, accept EVERY collateral duty thrown your way and ask for more. Appeal to everyone's good side and become a team player. You can NOT act like a E1/2/3/4, you need to act like a senior enlisted leader or junior officer to show you are capable of leading. The reason for this motivation is easy. If you Commanding Officer of your command does not see you exceeding your normal duties, you can't submit a USNA application.
3) Applying. So you picked up 5 or 6 collateral duties, you got a great evaluation and have a positive aura surrounding your name around your command. Time to route a request chit to apply. The number one thing that the admission board looks at when you apply is your CO's recommendation. If he does not submit one or refuses to sign for one, the board will not even look at the rest of your items.
Some CO's will not say no to anyone and will write a generic evaluation for you and most of the time the board can see through it and call bull****. CO's like my own denied 14 officer packages last year, I was one of two that were signed off. It is a gamble you have to take when enlisting that it might take you a few years to get the approval to apply.
You will be required to pass the candidate fitness test with the same standards as everyone else and pretty much fill out the same application as you are now; except two parts. One, you don't need a nomination! That is right, if you are under 23, have a decent SAT/ACT score and have kept your nose clean, the SECNAV will (hopefully) write you your nomination. He can appoint up to 170 people a year. And finally, you will need two extensive interviews with two active duty, unrestricted line officers at your command. I choose a LT that was a USNA grad and my XO(right under the CO in the chain of command) who was also a grad. Expect to answer tough questions that will make a job interview seem like eating pie. Questions asking why you should be appointed to attend the USNA as a enlisted leader and what you would do in various leadership positions are the norm. Interrogation was the best way I can describe these two interviews that lasted quite a few hours.
The main thing the boards look at is your SAT's/ACT's, your A/C school grades (if you finish in the top 10% you are good to go) and what you have done since you have been in the Navy. Separate yourself with your collateral duties, special awards doing your job and community service projects.
So as you see, it is quite a big pipeline to keep on track to reach your end-goal if you decide to enlist. You also come into this problem that pushes away 99.9% of those in the Navy from applying each year; you have to go backwards. In the "real Navy" you will have liberty to do whatever the hell you want after your working hours and you have far less restrictions than as a midshipmen. You will go from living like a normal human being, to being treated like a Plebe and you don't get any special cookies for making that choice.
My shipmates can't comprehend why I want to do it. Once you taste the sweet fruit of living on your own, many can no go back. I have stuck with my dream and could care less about my personal life, but just keep this in mind. Not many do it which is why those who are actual prior enlisted (I personally don't count Nuke transfers, they have never experienced the Navy) are a rare breed at Annapolis.
I know this was long, but I figure you should know what your getting yourself into before you make the plunge.
And if you want my opinion, I would not have applied to the Academy any other way. I am 1000000 times more mature than I was two years ago when I was out of high school and have experienced living in 9 different sates and 3 different countries in a little over two years. I have experienced situations that will make me appreciate and excel at the Academy that were incomprehensible to me just a few short years ago. Plus I like my shiny ribbons and tattoos.
