I'm currently in the NUPOC program. Here is my simple advice.
A's in EVERY class your freshman year. It will be nice to be well rounded and have come extracurricular activities on the chart, but as a Freshman, its all about grades. Your entire academic career will be set by the friends you make and the group of students you associate with. Go ahead and join a club or sport, however make sure that your focus is on your studies as a priority.
You will find that when you focus on your studies, the other sharp students will be your friends. You will study together, work together and grow up together. These guys and girls will likely not all join the navy, however learning to be among the sharp pack is vital to long term success. Not to mention, many of those college friends will go on to hold important positions in the future and become good contacts.
(Yes, I know there are some B/C students who end up as CEOs, but the majority of successes in the world come to those with the knowledge and the work ethic to succeed).
Most importantly, for your goal of joining NUPOC, you'll need the grades.
Feel free to search the boards, you can literally read my story and several others as we went through the NUPOC process. You will also get a chance to read about some who tried to get in and could not for one reason or another. Also check out
http://www.nukewarriors.com Work hard.