Your energy is communicated to the class. So is your enthusiasm. Let your face be animated. Eye contact is important. The use of mime and the use of your hands are encouraged.
Standing suggests energy but is more dominant.
Sitting is more informal but limits your vision and use of the blackboard. Sitting on the desk may be a compromise. Vary the pace of the class period. Be prepared to make creative changes while the class is in progress, if necessary. Occasionally repeat a lesson format a second time-it will go more smoothly and give you a sense of accomplishment.