As with all exercise you can't neglect the warmup. In aerobics, the warmup is meant to help prepare you for the workout. It tends to include stretching and movement of the major muscle groups.

Marching

Stand with your feet slightly apart and your arms loosely at your side. Begin by marching, or exaggerated walking, by bringing your knees up one at a time and returning them to the floor, alternating feet. You should take care not to stomp your feet down and not to raise your knees above a comfortable height. To increase the intensity, swing your arms as you march along, and be sure your abdominals are pulled in, your head is high, and your chest is open. Do this exercise for a couple of minutes until you feel slightly warmer.