Issue the following command with elevated rights or as root:
apt-get install x11vnc
Create the file /etc/init/x11vnc.conf with your favourite text editor and write this into it:
start on login-session-start
script
x11vnc -xkb -noxrecord -noxfixes -noxdamage -display :0 -auth /var/run/lightdm/root/:0 -forever -bg -o /var/log/x11vnc.log
end script