From your mobile device dashboard you will see all your calendar items. You will know you have been assigned a task or appointment by the red color for the calendar item. Open the calendar item and Accept or decline the task or appointment.
If you created the calendar task or appointment and assigned yourself as responsible party you will not be required to accept or decline the invitation. It will automatically be accepted for you.
The creator of the calendar item will be notified of your acceptance or if you declined the item.