You should go to your portal and the tab on the far right will say "Field Force Representative", click that. Email your Field Force Representative who's email is under that tab. Be respectful, address as Mr. or Mrs. or Ms., introduce yourself, tell him/her a little about yourself and ask if you have any questions or concerns ask. Most importantly be genuine. If you cannot find a field force representative in your area under that tab I would say email your state coordinator and notify him/her.
In addition to this you should do research into how each of your MOC (member of congress, representative/senators) does nominations and begin to apply there. You can find this information on your candidate portal under the nominations tab. While you're at it see if you are eligible for any service connected nominations.
Start building good relationships with your teachers as they will be evaluating you later in the process. Continue to get good grades and continue to do extracurriculars you enjoy (ex. varsity athletics), also seek leadership positions (ex. team captain). Start researching and training for the CFA now it will help you immensely in the long run.
Other than that hang tight and the application will open for you in the summer, I think around August. Once it opens get as much of it done as early as you can (make sure its quality). You should also apply to SLE if you are eligible, and get in touch with your RC asking how you can make your file more competitive.
Be the best you, good luck!