When you start working on an opportunity you can set yourself a set of tasks that relate directly to that opportunity.

Add a Task
Firstly, you might add a task to call the contact to qualify the lead with a due date, select  + or select Create New Task

Enter in the appropriate details

You can edit the entries to the drop down menus by clicking on the green circle icon.

When you have completed the task, you can select the check box for completed. 

Tasks are listed in a table in the footer of the opportunity details.