During class, your instructor will ask you to ride in first, second and third positions.
First position is seated. Your shoulders should be down and away from your ears, your hands should be loose and light on the corners of the handlebars and you should keep your back straight and chest open.
Second position is standing up with your hips hovering over the saddle. You should lift your hips two inches off the saddle and continue riding with your core tight and engaged to stabilize. In second position, your hands should be at the lower corners of the handlebars.
Third position is out of the saddle with your arms extended at the end of the handlebars. Maintain a long, straight back and, engage your core to stabilize and support your lower back.