Forth tab of a lead view is Tasks to view or add task
Tap on Add task to open a task create popup
This form contains start and end date, User drop down to assign task to user, Company dropdown to tag the company, title and notes
After scrolling you will find the Task Status dropdown, Contact type dropdown
At Related To dropdown, you can select any other lead to tag as related
Select the opportunity from dropdown to tag with opportunity name
Below it has two checkboxes to show the task in calendar and another is to enable a reminder
Tap on submit button to save the task