After setting up Debian, and for a week, I needed to enter my user name and my password to log on to my system. This is the default behavior of LightDM (the login manager of Debian) for maximum security.
As I got tired to enter my user name again and again, I researched for a way to have LightDM fill the last entered user name for me. As it turns out it is easy to setup.
All you have to do is edit the following file:
By typing in your terminal:
sudo nano /usr/share/lightdm/lightdm.conf.d/01_debian.conf
Then change the command greeter-hide-users from false to true.
Save the file (Ctrl+O) and exit (Ctrl+X)
Log out and you have fixed it.