How should I sit on a bike?

Comfortably! Don’t let anyone tell you that cycling should be uncomfortable. Sure, your legs and lungs might hurt, but your back and butt should not. A great starting point is to set the saddle height so your knee is straight and locked when you have the heel of your foot on the pedal. That way, you’ll have a few degrees of bend in the knee when you pedal, as you should, with the ball of your foot on the pedal axle. You should sit on the widest part of your saddle and be able to reach the bars without feeling stretched.