To install webmin on debian 8 just follow this instructions :

cd /root
wget http://www.webmin.com/jcameron-key.asc
apt-key add jcameron-key.asc
echo "deb http://download.webmin.com/download/repository sarge contrib" >> /etc/apt/sources.list
echo "deb http://webmin.mirror.somersettechsolutions.co.uk/repository sarge contrib" >> /etc/apt/sources.list
apt-get update
apt-get -y install webmin

If it’s too long for u, you can also just do this :

wget http://www.christophe-casalegno.com/tools/install_webmin.sh
chmod +x install_webmin.sh
./install_webmin.sh