From the Tasks list you can:
1) Complete an individual task
Click on the task itself to go to the customer page. When you are finished, making the call or sending the email, click 'Complete Task' at the top of the page.
2) Complete a series of tasks
Click on 'Start Tasks' on the tasks page.
This will pull up the first task in the list and as you complete each task, the next task on the list will appear. You can skip a task and come back to it later by clicking 'Skip'.