The pilot reported that prior to departing on a cross-country flight, that both of the aircraft's fuel tanks were filled with fuel. Additionally, the pilot stated that during flight she failed to switch fuel tanks. While talking to the airport's control tower and during the approach to the runway, the aircraft's engine lost power. The pilot elected to conduct a forced landing in a nearby field. The aircraft's landing gear collapsed and the aircraft was substantially damaged during the forced landing. A Federal Aviation Administration inspector who examined the airplane, reported that the right wing fuel tank appeared empty and that the fuel selector was on the right wing fuel tank position.