Check this out,
First you need to make a public - private key for your self.
gives clear instructions as to how to encrypt a document. If you don't like that follow this
There are GUI's available...seahorse, KGpg, GPA come to mind for Linux. WinPT for windows.
Everything you need is on the gnupg website. You might also have to install few libraries if you don't already have them but since you are using Mandrake, it would be easy to urpmi.