If in the open, move away from a tornado's path at a right angle. If there is no time to escape, lie flat in the nearest depression, such as a ditch or ravine.
Upper stories are unsafe.
If there is no time to descend, a closet or small room with stout walls, or an inside hallway will give some protection against flying debris. Otherwise, under heavy furniture must do. Additional protection is afforded by taking cover under heavy furniture or a workbench. Other basement possibilities are the smallest rooms with stout walls, or under a stairway. The first floor is safer than the second.