“i am new beginner at Icinga and i want a manual guide for step by step installation and configuration of icniga for monitoring of 20 devices using SNMP”
https://icinga.com/docs/icinga-2/latest/doc/01-about/
https://icinga.com/docs/icinga-2/latest/doc/02-installation/
https://icinga.com/docs/icinga-2/latest/doc/03-monitoring-basics/
https://icinga.com/docs/icinga-2/latest/doc/04-configuration/
https://icinga.com/docs/icinga-2/latest/doc/05-service-monitoring/
#snmp-monitoring
https://icinga.com/docs/icinga-2/latest/doc/07-agent-based-monitoring/
#snmp
That should get you pointed in the right directions.
If you get stuck, let us know how far you’ve got and what any problem is.
Have fun with icinga 
Antony.
