Welcome! My son talked of nothing but the CG from the age of 10; we began visiting with our nearest recruiters (two hours away) with him when he was 15.
He has been in the USCG now since August 2010, and is living an amazing life for young guy not yet 21.
Our advice, based on his experience: research the fitness standards, and start working out at levels appropriate to your development. Don't overdo weights-- your frame is still developing! Play sports, especially ones that involve endurance. Try not to get hurt; it complicates things later.
Get good grades; cultivate self-discipline early.
Stay out of trouble: the CG , unlike other military services, is law enforcement as welll as military. Their expectations are higher.
Go to www.gocoastguard.com and make an account. There's tons of good info, and on your 17th birthday, you may get an email from a recruiter inviting you to contact them.
Make an account on www.military.com as well. Take advantage of their online practice ASVABS later, maybe in your junior year.
Eat real food. Choose the company you keep with care. Have fun and be a kid till the last minute.
And visit here frequently. This is a great place to ask questions.