Run the following commands:
# cat > /etc/yum.repos.d/webmin.repo << EOF
[Webmin]
name=Webmin Distribution Neutral
#baseurl=http://download.webmin.com/download/yum
mirrorlist=http://download.webmin.com/download/yum/mirrorlist
enabled=1
EOF
Now run
# rpm --import http://www.webmin.com/jcameron-key.asc
Check for updates prior to installing webmin
# yum update
Install webmin
# yum install webmin
Now open port 10000 to access webmin, to open for all IP's for example
# iptables -I INPUT -p tcp --dport 10000 -j ACCEPT
# service iptables save
Be sure to lock down the webmin & port 10000 to spacific IP's for best security and to configure SSL as per comments below you will need to install SSL then enable it from the webmin GUI
# yum install perl-Net-SSLeay