Have you checked the excellent documentation at
https://www.nagios.org/documentation/ ?
It's one of the most popular subjects on the internet. Try a Search...
I see seven separate products there so "I am using nagios to monitor my environment" doesn't say much.
Please don't cause us to assume.
https://www.nagios.com/search/?q=ena...os.com&btnG=Go
If you still have issues, please let us know what you've read, what you've tried and
What product exactly?