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It's been well over twenty years since I was in your shoes so things may have changed a bit, but I have a few things you might consider.
1. Grades are very important so keep them up.
2. Study for the college admission's tests. There are a lot of study guides out there. Get more than one and start studying them. The sooner you start the better. Studying for these tests is a MUST. It's a good summer project. A High Score on the College entrance exams was what got me my scholarship.
3. Don't just apply for the Coast Guard Academy. Apply for all the military academies and for all the ROTC programs. If you don't get in to the CG Academy then you have some backup opportunities.
4. Athletics. Get fit. Join a Gym. Better yet participate in a lot of team sports.
5. Make some phone calls. This post is a good start, but call the Coast Guard Academy. Have them send you some information. If you live near the water go down and talk to some local Coast Guard officers. Get to know them. Part of the application process involves getting written references. A reference from a Coast Guard officer would look pretty good.
There are a couple of books you might look at:
1. Life Inside the Coast Guard Academy
2. How to win an ROTC Scholarship.
Go here to find links to them: