As in English, the passive voice in French is composed of a tense of the verb to be and a past participle.  Any transitive verb with a direct object can be made passive.  The active form, le chat mange la souris is made passive thus:  La souris est mangée par le chat.

The subject in the active sentence (le chat) becomes the object of the passive.  The object of the active (la souris) becomes the subject of the passive sentence preceded by "par."  The verb of the active sentence is changed into a past participle (mange becomes mangée, notice the agreement!) preceded by a form of être.

Elle est portée par Jean.  She is carried by John.
Elles ont été inspirées par Van Gogh.  They were inspired by Van Gogh.
Il avait été tué par les soldats français.  He had been killed by French soldiers.

If a verb takes an indirect object, it cannot be transformed into the passive voice.  In this case, on is used in the active construction.

On a donné un cadeau à ma mère.  My mother was given a present.
On lui a dit de retrouver le collectioneur au musée.  He was told to meet the collector at the museum.

Notice how pronominal verbs change from active to passive:  
Active: Je me suis réveillée.  I woke up.
Passive: J'ai été réveillée par quelque chose.  I was awakened by something.