First off, www.academyadmissions.com has more info than I can give on the current application process. You would be well served to read the information there.
As for general tips, sports are very good to be involved in. Try to keep your GPA as high as possible while taking the highest level classes you can. That should ensure a solid academic background, and possibly give you the chance to place into higher courses here (depending on how good your school is and how well you do). Get involved in activities where you can hold leadership possitions (could be team captain, class council, scouts, jrotc, club president, etc).
No, you do not have to be an engineer to be a pilot. I am a military history major at USAFA, and am trying to get a pilot slot. Your grades in college play a much greater role in your chances of becomming a pilot than your major does. The choices for what job you want (pilot, security forces, intel, civil engineering, etc) are based on your class rank in college. The 1st person in their class gets to choose first, the last person is pretty much stuck with whatever is left. Therefore, you should major in something you are good at and enjoy.