The best sitting position is towards the center with your body at about the 12th fret. The back should be straight and the arms should make 90 degree angles at the elbows. The forearms should be parallel with the ground. The upper arms should be in line with the torso. The seat should be adjusted so the thighs are either parallel with the ground or slightly angled towards the floor.
The feet should rest on the floor in the relaxed position. In the playing position, the left foot should hover over the A and B pedals (usually the first two at the left), with the heel touching the ground. The heel should be in front of the B pedal, to allow easy pivoting between the AB pedals and BC pedals. The right foot operates the volume pedal - more on that in a later lesson.
The hands should rest comfortably on the instrument in the relaxed position - more on the hands below.