You must say the word "Shotgun" to stake your claim on Shotgun. This must be done clearly and loud enough so that at least one other to-be occupant of the vehicle can hear you. No variations of this word are acceptable. After you have rightfully called Shotgun, you have exclusive rights to Shotgun for that ride. However, if no one hears you call Shotgun it is still fair game for everyone.
But that's just the beginning! There are many more rules to learn, such as the importance of having your shoes on when you yell "Shotgun!" and the crucial "hand on the door" rule. Link -via Bits and Pieces