1. Click on Activities on the top left corner of the screen. You can also press the Windows key as a shortcut.
2. Search for Settings and click on it.
3. On the left pane, scroll down or search for Users and click on it.
4. On the blue bar at the top of the page, click Unlock.
   
5. Then, enter your user password as the Administrator's password. In my case, it is 1211112299. Once that is done, you should notice that the blue bar is gone.
6. Now press Activities again, and search for Terminal. Click on it.
7. Type sudo whoami and see the results.
   
   The user megat is not in the sudoers file, we need to change that.
   
8. Type su root to go into root.
   
   Notice that megat@vbox changed to root@vbox.
   
9. Type sudo visudo /etc/sudoers to edit the sudoers file.
   
10. Search for the line # User privilege specification.
    
11. Add the line megat ALL=(ALL:ALL) ALL.
    
12. Save the file using Ctrl + O → Enter → Ctrl + X.
13. Type exit to exit root.
    
14. Type sudo whoami again and see the difference.
    

Congratulations, user megat now has sudo permissions!