One other thing.. if you are wanting to go to a service Academy, you'll have to be nominated by your State's U.S. Senators, a State U.S. Rep, and the Vice President of the United States - and maybe the President? Think it's on the web sites.
I *THINK* this usually starts around your Junior year, or probably after your Junior year, before you start your Senior year is probably the best time.
You need to visit the service Acadamey's web site(s) and see what the requirements are.
You have to contact those people early so they can "evaluate" your process.
http://www.usna.edu/Admissions/steps.htm
http://academyadmissions.com/admissions/
http://www.cga.edu/admissions/admissions.htm
Pretty sure the Coast Guard still doesn't require nominations for admissions. However, nomination or not, Coasties can't go to flight school straight out of the Academy either. You must do at least 1 yr to 18 months aboard a ship first. But, usually all those that apply eventually get in unless they give up applying or no longer qualified. And a year or so aboard a ship ain't bad. It's not like we deploy for 6 months.
Also, get that GPA up, move up in class ranking (you will with a higher GPA), participate in activities, student body government, sports, ROTC, academic clubs, volunteer at local Red Cross or Senior Citizen home - something - anything and everything. Take a look at the applications now, see the kinds of things they're looking for.. then work towards those.
Start studying and taking the SAT as soon as possible. Also, if you don't do great on the SAT, take the ACT instead. I've heard people who had trouble with SAT did better on ACT. Don't stop taking them. Work towards improving your score. You need a rather high score to be accepted. Selection is VERY competitive.
Everything counts in large amounts.
Good luck!