Many ballet instructors start the warm-up with light prancing in place for a couple of minutes. With your feet parallel, slowly and gently alternate raising and lowering your heels. Keep your knees soft and make sure you roll through your entire foot, including the toes. Prances can be done in place, moving forward, or moving backward.
Another good way to warm up for ballet is to perform rises at the barre. (Keep a tennis ball in your ballet bag for this one.) Stand at the barre with your feet parallel. Place a tennis ball between your feet, just below the ankles. Try to keep the ball in place as you slowly raise and lower your heels. Rises with a tennis ball will ensure that your body is properly aligned and ready for the class.