I am speaking as the father of two STEM DS's for whom HS level math and science (AP included) were easy. DS #1 is AROTC MS 4. DS #2 started out aspiring to USNA, changed to NROTC, got ROTC scholarship to Michigan, later DQ'ed for eye injury. I think their experiences would be instructive to you.
I am also writing about education and career choice; not about how to get into an SA.
I'm assuming by your user name that you would be shooting for the college graduating class of 2022, which would make you a HS sophomore. Your mentioning of Engineering tells me you are a math/science superstar and/or you just love machines and how things work.
1. You are very smart to start thinking about these things now and this forum is an excellent place to start. Always use the search tool to answer a question before starting another thread with your question.
2. Concentrate on the studies in front of you right now. As you get deeper into Math, Physics and Chemistry I predict you will find something you hate and something you love. We gave DS #2 subscriptions to Popular Science and Popular Mechanics. In those (after we tore out the creepy classified ads) he could finds things that tripped his trigger.
3. You will need to broaden your college wishlist. An SA would be great and keep that as a goal. The competition is stiff. It is also stiff for ROTC scholarships. There are any number of things beyond your control in the application process. You may even just simply change your mind. You may have a disqualifying injury.
DS #2 was kicked in the eye with a soccer ball two months before receiveing his scholarship and was medically DQ'ed. When he lost his NROTC scholarship, the cost to attend U Mich went from $10-12k to $40k. One of his back-ups was in-state Big 10 U, cost $0. He is Mech E sophomore, works in a physics lab and couldn't be happier.
4. Understand that you will change over the next several years in ways that you can't imagine. Some of the changes will come from within as you mature and some will be situational. Some are gradual and some are sudden.
When DS #1 began compiling a list of schools, he was deadset on finding the best Chem E programs. He shared the list with a good family friend who chairs an engineering department at the local Big 10 U. He told DS to think no more narrowly than "I love Chemistry" or "lasers fascinate me". Until you actually get into the guts of engineering, which really doesn't begin until Sophomore year, you have no idea what you are getting into. Engineering can be extremely boring and tedious, so you better love what you are doing.
5. Engineering is by far the toughest college major. Almost all programs leave virtually no opportunity for electives. At an SA or in ROTC you are on a four year clock, so a ninth and tenth semester are not really an option.
DS #1 hated the E part of Chem E for all the reasons mentioned. He switched his major to Chemistry. He carried in quite a few credits from HS which would have allowed him to easily finish in four years, but he wanted more academic freedom and less time management stress with his ROTC commitments. He was able to take Arabic, enough Computer Science for a minor, and graduate on time.
He got his first choice of Active Duty of Signal Corps with the opportunity to work in the brand new Cyber Branch.
I will let others remind you that an SA or ROTC at a civilian university is not just another college experience. If you are not absolutely committed to serving in the military at the military's pleasure, life can be very unpleasant before and after graduation/commissioning.
Just ask those AFA graduates who aspired to fly jets and just found out they will be missileers working 200 feet underneath the North Dakota prairie.
