Before you begin verify you server and client configuration. From the server test client connection.
To setup server, see http://magnus-k-karlsson.blogspot.se/2014/01/install-nagios-core-35-on-rhel-6-from.html.
Client Nagios Plugins Configuration
First decide what you want to monitor. Then find a plugin that do the job. The standard plugins are documented here (http://nagios-plugins.org/doc/man/index.html).
You could also get help documentation for a specific plugin by executing it with option '--help'.
After you have tested it and is satisfied, you now define it as a nagios command.
Restart nrpe to let new configuration take effect.
Server Nagios Plugins Configuration
From the server test that you can call the client nrpe command.
If this did not work, go back to the prerequisites and verify you server and client installation. If good, lets continue and define the nrpe command on the server.
Then we need to define a service that will run this command.
Finally you will need to define a host, if you have not already done that.
Before we restart the nagios server, you can test you new configuration
If everything is fine, you can now restart you nagios server.
Open you nagios web admin and login in and check you new service.