Webmin is a web-based interface for system administration for Unix. Using any modern web browser, you can setup user accounts, Apache, DNS, file sharing and much more. Webmin removes the need to manually edit Unix configuration files like /etc/passwd, and lets you manage a system from the console or remotely. 

Below simplest step to install webmin on Centos 7.

Pre install, you need nano and wget first. by typing:
sudo yum install nano
sudo yum install wget

Now Let`s install:
1. Update system first
yum -y update

2. install depedency
yum -y install perl perl-Net-SSLeay openssl perl-IO-Tty perl-Encode-Detect

3. Add new webmin repo
nano /etc/yum.repos.d/webmin.repo

and paste this
name=Webmin Distribution Neutral

Then Ctrl X and Y to save

4. Now fetch and install the GPG key which was used to sign the packages by running the following command.
wget http://www.webmin.com/jcameron-key.asc
sudo rpm –import jcameron-key.asc

5. Install Webmin ready
sudo yum install webmin

6. Allow port 10000 for webmin
sudo yum install webmin
firewall-cmd –zone=public –add-port=10000/tcp –permanent

7. reload firewall
firewall-cmd –reload

and you done.