Hey Payton,
I'm assuming that you mean you'll be entering your junior year of HS starting in a few months. A great option would be to apply to a service academy- look into the application process now and also apply for a summer seminar program at the academy if you choose to go that route. I hate to say it, but just due to how the number ratios play out, your best chance at being a pilot is to go the Air Force route. (threw up in my mouth a little bit saying that) Navy is a great choice too, but to get a pilot spot, you're really going to have to work hard for it- like be in the top third of your class or better.
Another option is to apply for a ROTC scholarship and go to a college that participates in the ROTC program.
GPA is big- shoot for 3.5 or better...probably much better. Also, participation in sports teams and volunteering in your community is huge too in getting accepted into a commissioning program. You're going to need good SAT and/or ACT scores as well. For the ACT, I'd say 27 is bare minimum, 32 or better competitive.
Once in a college commissioning program, honesty is, your field of study is not that important as far as getting a pilot slot goes. Pick something that you enjoy studying- the biggest thing is maintaining a good GPA. Also, consider that more than half of pilots do not retire from military service unfortunately- you should pick a degree that will support you in your second career. Computer science is very needed these days, engineering is a great choice too.
Getting a civilian pilot license won't help you get a pilot slot and will only help you marginally in the very beginning of primary flight school. In my opinon, it is not worth the time, energy and effort to get one just for the sake of helping your chances. You can get much of the same basic result by reading a private pilot book and getting familiar with basic flying concepts on a computer based flight simulator.
Flight school students with civilian flight experience were better at radios and struggled less with basic airwork, but after the first few weeks of flight school, that edge levels off as the students with no experience come up to speed. If anything, there are a lot of bad habits that have to be unlearned. It might help with a first impression with flight school instructors and you'll seem a bit more competent and a quick learner, but most kids with flight experience also brought a bit of arrogance with them that tended to negate that first part.
Flying in general is awesome, and I encourage you to get some flight experience if it's for your own personal enjoyment, but again, if it's just for the sake of getting an edge in flight school, the benefit is minimal.
Best of luck.
"Beaker"