I have installed nagios but i cant open in the web interface? web just give me notice "Unable to connect" sorry for my ask because i'm newbie user linux, please help

Welcome to LQ, hope you like it here.

Please note configuring some services can be seen as a "right of passage", as they require you to do more than just blithely install and start that service. You need to read and understand the (quite extensive) documentation to configure the service to your liking, verify configuration validity using any configtest or pre-flight checks the service offers and check stdout / stderr and log files for clues.

So best start by describing which Linux distribution you use, how you installed Nagios, how you configured it, what stdout / stderr, /var/log/messages and Nagios log files (if any) return when you start the service as well as verifying with ps / pgrep / netstat the service runs, taking into account any web server configuration and ACLs like firewall, /etc/hosts.allow or MAC (like SELinux).

thanks for your help, i apreciate it

I haz a toaster.
Bullet 1. Need more details please.