I can give a brief outline for you, but i do second what was said above, there are many posters on this website that carry soo much info! (that's how i learned most of it as well)
sophmore-
good grades, shoot for the highest possible gpa,take some pre-ib or honors courses.
during sophmore year, i was in marching band,girl scouts, coached baseball/soccer, ASB, NHS, and in 4 honors classes. my point is too keep your self busy and challenged,always!
with girl scouts, leadership is a core value, you will want to have something that shows your a leader.
Junior:
september try out the sat and act (take them "cold",or without preparation to see how you do) why? you get your scores in october, and you can start studying to take them again.
october take the PSAT or PLAN (you will need these scores for the summer seminar.
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december 1st- the first summer seminar application opens (usafa) usma is dec15, usna is feb1,and uscga is also the 1st i believe. you will want to complete these as early as possible,it helps.
feb take the sat again
march start looking into rotc or college options/apply (march)
marchstart looking into local rep/congressman dates for nomination applicaitons
april keep up the grades! and start asking teachers if they will write letters of rec for you over summer (you'll need their emails)
may take the sat/act again
in all of this, don't forget to stay in shape! you're going to have to take a physical test (1 mile run, situps,pushups,pullups,basketball throw and shuttle run)
You should get your candidate package in may? or a little into june. then the fun begins
(this is where i am at in the process)
during the summer you will apply for nominations, take the CFA (physical test), and finish the application on your end.
The earlier you get the application done, the better!
if you have any questions, pm me, or look on this site.
it is a wealth of information.
I think it's great your looking into this so early. good luck with everything!