1. setup dag repository
Edit /etc/yum.repos.d/CentOS-Base.repo adding these config:

[dag]
name=Dag RPM Repository for Red Hat Enterprise Linux
baseurl=http://apt.sw.be/redhat/el$releasever/en/$basearch/dag
gpgcheck=1
enabled=1
gpgkey=http://dag.wieers.com/packages/RPM-GPG-KEY.dag.txt


2. install nagios

yum install nagios nagios-plugins


3. Setting Up The Service
Edit /etc/nagios/cgi.cfg, doing these settings:

use_authentication=1
authorized_for_system_information=nagiosadmin
authorized_for_configuration_information=nagiosadmin
authorized_for_system_commands=nagiosadmin
authorized_for_all_services=nagiosadmin
authorized_for_all_hosts=nagiosadmin
authorized_for_all_service_commands=nagiosadmin
authorized_for_all_host_commands=nagiosadmin


Start up the service:

service nagios start


4. Setting Up The Web Interface
Setting up a user:

htpasswd -c /etc/nagios/htpasswd.users nagios


There already has a /etc/httpd/conf.d/nagios.conf installed, so you just need to restart apache:

/etc/init.d/apache reload


Now you can visit the nagios web interface at:
http://localhost/nagios