What the important thing we need before we can setup The Mountain Lion Server application

Make a backup of your Mac before setting up Server app, just in case you want to go back to a previous status when Server wasn't installed. You can make the backup with Time Machine > http://support.apple.com/kb/HT1427

Apart from that, nothing more. Note that OS X Server is OS X Mountain Lion but with Server app, so you just have to stop services and delete the Server app if you want to go back to Mountain Lion

I'd ask what requirements you're looking to fulfill by using OS X Server on a MacBook system, as that's not a typical hardware configuration for a server.

OS X Server and most other server operating systems expect static IP addresses and full-time availability, where a MacBook tends to be mobile and to thus have a variety of dynamic DHCP IP addresses and networks, and MacBook systems also tend to be closed and hibernating at random times.

It's certainly possible to use this hardware as a server (and it can be useful when you're learning how to manage a server and have a "scratch" MacBook available), but the hardware is not necessarily a good fit.

As you add services, you'll want to have DNS services established and verified, and — on a typical NAT'd local area network with no other LAN-local DNS services — that involves configuring DNS services on OS X Server itself. DNS is a prerequisite for various other OS X Server services.

But yes, do have a disk backup, either Time Machine, or as an external disk backup, or however you preserve your data. Skimming the OS X Server manuals can help save some time figuring out the platform, too.