and download from it or browse some guides thereon. You should complete the Linux Intro guide available there. You will have a better understanding of what to do next or where to go next from there.
Mail Server and other Servers require that you know at least basic commands and working knowledge of vi or vim or some other editor available on your Unix / Linux distro.
Try to grasp that knowledge first and experiment with the basics and then you are free to move anywhere in the Linux World.