When doing step-ups for the first time, the dumbbell side step-up using a low step is a good place to start. A progression over time to increase the range of motion involves standing in front of a step as demonstrated above and raising the height of the step as strength increases. The greater the range of motion, the more muscle you recruit. In general, the higher the step, the more the hamstrings and glutes become involved.
The key with step-ups is to determine the appropriate height first. This is the height where proper form is exhibited. There should be no assistance from the opposite (non-working) leg, the trunk should remain as vertical as possible, and the knee of the working leg should track over the middle toes. Keep raising the step height until form deteriorates, and then go back one level. This is where you should begin.
Aerobic steps and plyometric boxes can be used, but one of the best units on the market for step-ups is the C-206 Leg Platform by Atlantis. It allows you to adjust the height from 10 inches to 34 inches in 2-inch increments to accommodate various levels of strength. This is the unit that we use at our training facility and the one we recommend to others.