In order to be a pilot for Delta or American you will need a college degree to be competitive. Therefore high school grades are important. Besides taking your required courses, I recommend taking the electives that are most interesting to YOU, Not only will taking classes you like result in better grades but it will help you discover where your true interests are.
Flying related classes that may interest you are geography, weather, aerodynamics, physics, navigation etc. You could also look for a flying ground school and perhaps a summer job at an airport or FBO. Of course, flying lessons will start you down the road to an aviation career and organizations like AOPA and the Civil Air Patrol have education resources for kids your age.
As for the military, it is a great way to go that will give you the best training while you earn a salary. However, I would only suggest the military if serving your country and flying military aircraft while conducting military operations really interests you. Don't do it just to get an airline job because military duty requires a commitment and lifestyle that isn't for everyone.
Good luck to you. I am sure you will accomplish anything you want if you give it your best effort.