This tutorial will help you install Proxmox 5 on a freshly installed Debian 9.
It was tested on a dedicated server at hetzner.
Fisrt things first. Create a custom repository to pickup Proxmox related packages:
echo "deb http://download.proxmox.com/debian/pve stretch pve-no-subscription" > /etc/apt/sources.list.d/pve-install-repo.list
wget http://download.proxmox.com/debian/proxmox-ve-release-5.x.gpg -O /etc/apt/trusted.gpg.d/proxmox-ve-release-5.x.gpg