How do you determine the positive axis direction of the third axis in the Cartesian coordinate system? Which direction of rotation is positive for torques? These two questions can be answered with the "right-hand rules".
Right hand rule
In a coordinate system with the thumb as the x-axis and the index finger as the y-axis, the middle finger points in the direction of the z-axis.
Right thumb rule
When the thumb is pointing in the positive axial direction, the curvature of the fingers indicates the positive sense of rotation of moments.