Downloading OS X Server...

First, make a backup of your Mac before installing OS X Server. OS X Server is sold as an application (and it's an application on Mountain Lion) in the Mac App Store, so go there, purchase it and the download will start. When it finishes, the Server installer will set up OS X to use the server options.

If you want to uninstall Server, just open Launchpad, hold Option key and press X key above Server icon, so it will be removed just as any other app

My only suggestion is if you need some help to set it up or you don't know what to do on a step, you have a review and a how-to on a lot of sites, but I like this review > http://arstechnica.com/apple/2012/07/the-server-simplified-a-power-users-guide-t o-os-x-server/