Linux 7.0

Hi, my name is Chris and I just started using LINUX. Now my problem I have to install a very save server. The server should work as a file-server which is also WWW-router, firewall and Mailserver for a network of 25 workstations with Windows NT. 5 Days ago I started working in LINUX but my employer wants me to create a server that is saver than a very save NT-system. I'm not able to create such a system by myself!! Please Help me Greetings Chris