prepare for sth.为某事做准备 be prepared to do /for sth.对（做）某事有所准备

1 prepare sb / oneself for sth -------是及物动词,主动语态.
Let's prepare him for the journey
2 prepare for sth ------不及物动词.
The whole class is working hard preparing for the coming exams
3 be prepared to do /for sth.是prepare sb / oneself for sth 的被动语态结构
I am sure the police are prepared for trouble in this sense.