If you want to log a call, meeting or other past interaction, use this feature. From the Opportunity record, click on "Log A Note" in the side panel on the right hand side.

Include relevant details in the Comments field (1), set the Due Date field (2) to the day the interaction took place and search for a contact to relate to the record in the Name field (3).

After you hit save, the note will be linked to the opportunity record and the contact record you chose in the activities panel. At this point you can also attach files to the note record by clicking into the note record and choosing "Files" from the related tab.

If you want to specify a task assigned to a specific user, use this feature. From the Opportunity record, click on "New Task" in the side panel on the right hand side.

Enter a Subject, then choose a Due Date, assign to a User (it'll be you by default but you can change it to be any internal Salesforce user) and set a Status.  Then look up and choose a contact to relate the task to and include any relevant information in the Comments field. Finally, choose the priority status and assign a type.

After you hit save, the task will be assigned to the user as well as linked to the opportunity record and the contact record you chose in the activities panel.