Normally, the passive voice is something which is learnt aferwards, but I think that it is easier than it seems since it has the same structure as in Spanish. What you have to bear in mind is that the passive voice is formed with the past participle of the main verb and the verb «to be» in the same form as the main verb in the active sentence. So that, if the main verb is in present continuous in the active sentence, the verb «to be» will be in present continuous in the passive structure, and the main verb in the past participle form. Look at the next example:
Peter is writing a book. A book is being written by Peter.
Anyway, here you are some links with explanations and exercises for you to practise.