On your Dashboard, on the right-hand side, you will see a Task block. If you have a Task assigned to you, you will receive a notification and see it here:
When you click on the Task, it will open the Task Detail. To show that you successfully completed your Task, click the Purple Mark Complete button in the top right corner.
If any comments are necessary, make sure to leave them using the Leave a note/comment/review box at the bottom of the Task Detail.