Former Marine here (13 years), former DI, etc...
Advice for boot camp: Listen carefully, do as you are told, move fast, be motivated.
Pay attention to the instructions passed to you, do not do extra, but do what you are told to do quickly and do the best possible job that you can do. Everything you do, move with a purpose...you'll stand out this way. Motivation, even though sometimes false, is contagous. When the others around you are moving slow or are feeling a little homesick or whatever...your motivation could be all that is needed to pick them up. Keep those around you motivated, and your time will go by quickly and much easier.
Even if you are not the most physically fit, doing these things could get you noticed, possibly get you in a position of leadership, and maybe even get you an early promotion out of boot camp!
As for the rest of your time in the service: Become the duty expert in your field. That means learn from others, and from their mistakes. There is no replacement for hands on experience.