Go to System Settings >> User Accounts and switch the Automatic Login to OFF.
Now set your preferences in the following sections:
System Settings >> Brightness & Lock
System Settings >> Security & Privacy >> Security
If not already existing, set a user-password, replace 'your-user-name' with your real user-name:
sudo passwd your-user-name
Open the following file:
gksudo gedit /etc/lightdm/lightdm.conf
Remove or comment out (#) the line: autologin-user
Save and Exit
Reinstall gnome-screensaver, update and reboot your system:
sudo apt-get install --reinstall gnome-screensaver
sudo apt-get update
sudo apt-get upgrade
sudo reboot