The passive form is used when the person towards whom the action is performed becomes the subject of the sentence instead of being the object.
If we want to rephrase this sentence in the passive we have to make the object book the subject of the sentence: the book is read by me.
In Japanese the principle is the same: we want to make the person towards whom the action is performed the subject of the sentence.
Since the subject and object of the sentence are expressed using particles, it will be necessary to change the order of the particles as well as conjugate the verb in the passive form.
The passive form is simple to conjugate in Japanese. We must make a distinction between Ichidan verbs, Godan verbs, and irregular verbs.
To conjugate the passive form of the Godan verbs we have to modify the last Hiragana in the Base B1 (Negative) and add ~れる.
In this example we see how the verb 飲(の) む (drink) is conjugated in its Base B1 by removing む mu and adding ま ma, that is the Hiragana symbol that is part of the same series as む mu (both start with the character m) but ending in a: ま ma.