General knowledge of Linux is required.
This tip has been tested on Debian Squeeze with Apache 2.2.16.
Tonight i deployed a brand new Debian server with Apache and i got an annoying, but essentially harmless, warning every time i restarted Apache.
The error was Could not reliably determine the server's fully qualified domain name, using 172.25.27.10 for ServerName
The solution, after a little googling and a few false suggestions, was really simple.
Edit /etc/apache2/httpd.conf with your favorite text editor. I use nano so my command was (note that i use sudo even on Debian) sudo nano /etc/apache2/httpd.conf
Add a new line which reads Servername [your hostname], for example Servername webserver01
Restart Apache and everything should be fine.
Feel free to ask any questions or add comments regarding this post in the comments. I'll try and answer best i can, promise.