I.Preparation

Install epel and remi repositories. Reference link here Click Here

Make sure Apache , PHP and MySQL have been installed in the server

Reference link here click here 

Stop iptables service

# service iptables stop

# chkconfig iptables off

II.Proceeding 

1. Setup on server

Install nagios and accessory packages via yum command

# yum -y install nagios nagios-plugins-all nagios-plugins-nrpe nrpe

Start nagios and run it together with the system 

# service nagios start
# chkconfig nagios on

Set a password for nagiosadmin user used to login 

# htpasswd -c /etc/nagios/passwd nagiosadmin

If you don't use nagiosadmin user, you need to change and re-configure the file  /etc/nagios/cgi.cfg 

Here you can visit http://IP-vps/nagios