How could I best prepare myself and what direction is best for me?

I have wanted to serve my country for a long time. I am currently 15 and entering my sopre year of high school. I get great grades (almost always 4.0's) and take all of the honors cles I can get my hands on. I will be taking all sorts of AP courses in the next two years as well. My schooling has a large emphasis on math and science, specifically the harder varieties of it. I am moderately physically fit, and also have a black belt in Tae Kwon Do. I also play airsoft on a military simulation styled team, where we use military tactics and skills to achieve our goals, and play for whole days sometimes. I have wanted to join the Marines for a long time, as my family has a lot of history with them (as well as the Navy). I have always been impressed by their status as Marines. However, a guy I know has been trying to get me to join the CAP and consider the Air Force in the future because he says that it has the best options for college (not to mention with CAP service you can enter the Air Force as an E-3 or E-4, I forgot which). I have always thought that Infantry was the most exciting MOS to choose, with all of the rounds you get to fire, the bonds you form, the time outdoors, and living off of what you can carry on your back for days at a time. I have also been impressed by all of the more elite platoons like STA and Recon. One of my friends (he is almost shipping off to Marine Corps Basic Training) told me I could enlist in the DEP when I was 17 and go to boot camp after I graduated. I know that even if I decided to stay in for a longer time than the basic enlistment, I would want to make sure I was able to get a 4 year college degree somehow. I figure that I could do the first two years of it during my first enlistment but how could I do the last two? If I could get some advice on anything I have said that would be great. Thanks in advance.