Tasks are the individual bits of work that combine together to complete a goal, and should be used to break up the different parts of a goal. While Goals generally have stages of completion, a task is either incomplete or complete, and doesn't consist of stages. This should assist as a rough guide when deciding whether to create a sub-goal or task.
Once you have added a Task it will display in your Task list in the Goal Hub for this Goal (and also your Task list widget on your Dashboard), alongside all your other Tasks relating to the Goal. To find out how to create a task click here.