Auto-login enable on turn on system

First method:
Go to “System -> Administration -> Users and Groups” on the top panel.
Click on the user you want to set up auto-login for and click “change” on the Password field.

If “Don’t ask for password on login.” is checked, uncheck it.

Second method:
Edit /usr/share/lightdm/lightdm.conf.d/60-lightdm-gtk-greeter.conf.
NB: if you don't have that file in your system, try with /etc/lightdm/lightdm.conf.

You can edit that file by running sudo gedit /usr/share/lightdm/lightdm.conf.d/60-lightdm-gtk-greeter.conf (you can change gedit by your favorite editor).
The output of the file will look like this:

[SeatDefaults]
greeter-session=lightdm-gtk-greeter
autologin-user=username  

To enable password on login, comment the line: autologin-user=username by adding # in front of it.
Finally, save and exit.

Credit: How to Enable Automatic Login in Ubuntu