After pressing the startup button of your computer, you want to auto login and end up in i3.
These settings are manipulated in /etc/lightdm/lighdm.conf.
We have a menu that will make it easy to edit it. This menu is going to edit this file as sudo otherwise we can not change it.
Sublime-text can edit these protected files as well without starting sublime-text with gksudo. It will prompt for the password if you want to save the changes.
Follow the tutorial to change the settings.