I read this document that in this page :
but I can not understand almost of it ,, For example here is part of the file :
This variable specifies where Nagios should create its main log file. This should be the first variable that you define in your configuration file, as Nagios will try to write errors that it finds in the rest of your configuration data to this file. If you have log rotation enabled, this file will automatically be rotated every hour, day, week, or month.
I went to this directory /usr/local/nagios/var/
But I did not find file named nagios.log is it means I have to create it or what ???
Please if there is any body guide me to modify existing samples files , i do not want complex modification i want only monitor CPU, Memory , HD and Drawing the Networks Map .....