To get started after installing CentOS7 you will need to follow these steps:
First Update CentOS
yum update
Install wget
yum install wget
Download Virtualmin
wget http://software.virtualmin.com/gpl/scripts/install.sh
Install Webmin
/bin/sh install.sh
You should now be able to access Virtualmin Control panel via https > your domain > port 10000
To install Webmin on Debian Linux 9.5 you can follow these commands, (accept any prompts)
wget http://prdownloads.sourceforge.net/webadmin/webmin_1.890_all.deb
apt-get install perl libnet-ssleay-perl openssl libauthen-pam-perl libpam-runtime libio-pty-perl apt-show-versions python
dpkg --install webmin_1.890_all.deb
That's it, basic setup is now complete.
You can now access your server using the default https:// {youIPaddressOrServerAddress} :10000
Be sure to properly secure your server before putting it into production.