THE ACFT HAD BEEN REFUELED AT A NRBY ARPT BEFORE THIS PLT STARTED HIS FLT. HE TOOK OFF & FLEW THE ACFT FROM OKLAHOMA CITY TO TISHOMINGO, OK, WITH THE FUEL SELECTOR PSND TO THE RIGHT TANK (THE TANK THAT WAS LAST USED BY THE PREVIOUS PLT).ON THE RTRN FLT TO OKLAHOMA CITY AT NIGHT, THE PLT'S FATHER NOTICED THE RIGHT GAGE INDCD EMPTY WHILE THE LEFT INDCD FULL. PLT COULD NOT FIND THE FUEL SELECTOR VALVE & REQUESTED HELP FROM ARTCC. BEFORE THEY COULD HELP HIM, THE ENG LOST POWER. THERE HAPPENED TO BE A PLT AT ARTCC, WHO WAS FAMILIAR WITH THE PA-28. BY THE TIME HE WAS FOUND & INFO WAS PASSED TO THE PLT OF N7593J, THE ACFT HAD NRLY DSCNDD TO THE GROUND. AS THE PLT WAS REACHING FOR THE FUEL SELECTOR, THE ACFT HIT TREES & CRASHED. LATER, THE PLT SAID HE HAD NEVER OPERD THE LANDING GEAR OR FUEL SELECTOR DRG HIS CHECKOUT IN THE AIRCRAFT. HE THOUGHT THE FUEL SELECTOR WAS ON THE FLOOR, BETWEEN THE SEATS, BUT IT WAS LOCATED ON THE LEFT WALL OF THE COCKPIT, NEAR THE PLT'S LEFT KNEE. THE FUEL SELECTOR WAS FOUND PSND TO THE EMPTY RIGHT TANK.