You have a server running a Debian 8 and Nginx, and now you want to install Munin to monitorize it... Here you are steps to achieve it.
Ensure that the system is up to date before you start to install Munin, run:
apt-get update
apt-get upgrade
Install Munin:
apt-get install munin munin-node munin-plugins-extra
Création d’un lien symbolique pour la partie serveur web
ln -s /var/cache/munin/www /var/www/munin