The pilot and three passengers departed the airport in a single-engine airplane. During the climb, when the airplane was about 2,100 feet mean sea level, the engine lost power. The pilot reported that he performed the emergency procedures, but did not think to check the fuel selector. The pilot elected to conduct a force landing in a nearby field. During the landing the airplane’s firewall and left wing sustained substantial damage. The pilot also reported that the fuel selector was on an auxiliary tank that did not contain any fuel. He added that the fuel selector was easy to move inadvertently, and that a passenger’s foot may have moved the selector before takeoff.