How should I prepare for Army AIT for 31b Military Police?

Best thing to do is ensure you are eating right, staying in shape, getting enough sleep, and put your personal affairs in order. Pay attention to what the NCOs who have responded have said,

If you can pass high school, you can pass the "academic" portions of MP school. Everything else is mental. Everyone wants to give up at some point, especially at the beginning of basic. You get there and you're like, "oh, heck, what did I get myself into?" Bit you can do it. You just have to decide to do it. You shouldn't need to prepare for it, except for physical fitness. Remember that it will all be over before you know it, but remember what you learn.

Just remember, it's 90% mental, but the you that goes in will not be the you that comes out. The Drill Sergeants really do want you to succeed, but it's their job to break you down so they can mold you into a Soldier.