halo 3 campaign help

Iwant to play halo 3 online ,But can never find a person to play with howdoI .match up with poeple in campaign .online.? sorry im a noob

as far as i know you have to call your friends in you cant join a room with random people for co-op play.dejo256

Its is set on xboxlive open party ,but there is no matchmaking to play with other poeple .It goes straight to the game ....