When making sentences, if you are given parts to a sentence then look at the pieces of the sentence and then read it in different orders. One way is bound to sound right. If this isn't working out for you look at which pieces have a capital letter and which have a full stop.
A passive sentence is a sentence where the thing that is having the action done to it is the main focus of the sentence.
The boy kicked the ball.
The ball was kicked (by the boy).
The second sentence is the passive sentence. The part in brackets doesn't need to be written.