There has to be a plan starting in 8th grade.
If youre a boy, Eagle scout is nearly a requirement. Play a sport. Music is good. Be a student officer of some kind. Extra curriculars matter a ton to byu admin. And... The reason I said 8th grade, habits have to be in place by then. Cause in 9th grade the GPA counts. I know kids who screwed around in the 9th grade (because it wasn't in high school they sorta blew it off), well... can't do that, they sunk their byu plan before they got to high school.
Gotta keep your eye on the ball every quarter from 9th grade till December of the senior year.
And take the ACT a few times till you get your score you need, and take a sylvan course if you need it.