Hey brother, after going through it just over a year ago now, you honestly don't need to know anything going into it.
Maybe learn how to sew (sounds stupid, but TRUST ME, it'll come in handy once you have to label all of your kit lol), learn to properly polish boots, iron.
It'll make things easier for yourself during your really busy weeks.
Someone commented before, but they are correct, you will be taught everything you need to know by qualified instructors, that I 100% agree with.
Good luck and think of the 12 weeks as the hardest probation to any job you'll have