Security Step by Step- Keeping your server safe!

[happz]

After looking at my logs (centos) .../var/logs/mail.log and all the other attempts to break in or use my server for bad, I think it imperative that server owners that are new to webservers (like me) get a step by step on securing their servers. By the way..what is the file dead.letter? found in at /root/dead.letter?

So, would it be possible to compile this howto? Like "the 5 things you must do before going live/production" :

1. Turn on your firewall (don't know if this is needed but just an example)
2. Configure host.deny (I've seen something like this but nothing says that you must)

I would like to know what is a must in order to sleep well at night... knowing that my server is not being hacked.

happz