When i went thru the Nukes didn't get any speical slots. You can apply for it when you are in A-school (be it ET,EM or MM), they'll give you a physical (see how fast you can run, do sit ups and pull ups and the like), then send it on up. Oh and you'll have an interview with the Captain of A-school, he's to be your sponsor for the application, if he likes you your golden if not, well there's always next year.
Do not just limit yourself to the USNA, there is that new STA-21 thingy, which we didn't have when I was in. Surf the net and get all the info you can. While in school do your best, put in extra hours of study and maybe even help out a struggling classmate, keep your uniforms spotless and a good haircut. These things will get you noticed as a team player and someone MIGHT put in a good word for you. Some of the slackers there might harrass you there, due to looking like a suck up but, hey if it gets you those butter bars then it's all good, right?
good luck
dave