I would add the ability to keep your temper in check. Someone who lets others get under their skin easily is going to be involved in a lot of hassles he doesn‘t need. Learn (if you haven‘t already) to laugh stuff off and don‘t take every comment about you as an unholy stain on your sense of personal honour that must be immediately exonerated. Being in the Army doesn‘t mean you have to act like a brute or a jackass. The comments above about education is probably the best advice anyone could have given. Keep your grades up in school and don‘t let the Army take over your life. Again, there is plenty of time for that later in your career - your first duty is to finish school. And hey, guess what - the Army looks at it that way, too. You‘ll find that your unit will be very sympathetic to you missing an exercise or whatever if you explain you have finals or midterms coming up.
So honesty is tied in here, too, and is one other essential. Don‘t tell your section commander you have exams and can‘t go anywhere on the weekend, then get caught at a hockey game or at the mall. And above all, live up to your commitments and show your NCOs that you are capable of acting like an adult.
I‘ll give an example of what I mean. I‘ve been a company clerk in a rifle company for the last two training years. Occassionally some private will pull a bonehead stunt and get himself in trouble. Fine enough, soldiers to that. Without abusing any confidentiality, I can tell you that nothing aggravates the CSM and the platoon NCOs more than a soldier who then disappears for weeks at a time after learning he‘s being charged. Usually, the charge is simple - someone forgets to shave one day, or takes off without getting permission from a training day. In our company, the CSM takes great pains to ensure the soldier is not only made aware of his rights, but that he is at the same time made aware of how serious a matter military discipline is, but at the same time put at ease and told that there is nothing personal, that the NCOs don‘t have charge parades and summary trials because they enjoy it, but because it‘s part of the way the Army works.
The best thing to do in these cases is for the soldier to be honest about what he did, and then show up for his trial and get it over with. Organizing summary trials is a huge administrative process - I‘ve typed up some of the paperwork that goes with these, but the investigation itself, which I have nothing to do with, is also a royal pain for the NCOs.
So be honest, if you make a mistake, own up to it, and just keep in mind what is important - your grades at school, your doing what you are told, and ask yourself before you do something silly - is this what my sergeant or my sergeant major would want me to do?
And have fun - keep your nose clean, and we‘ll look forward to hearing about your experiences in the Militia once you‘re old enough.