Tell me know!

Please tell me know what file need to configure for running Nagios as basic.
Example i want monitor status of Web Server and if it change status to down, Nagios 'll send to me a mail ( message) , What should i need do?

OK, it’s obvious to me you haven’t spent nearly enough time reading the docs nor the sample-config files. It becomes fairly obvious after you do so.
In other words, define a host, define a service, define a hostgroup, define a contact, define a contactgroup and so on. The docs clearly explain what to configure and how, and also on how to test your configs.