When I received my Mac, I was so excited that I chose some retarded name which was bothering me for some time.

Mac has two different names, The Computer name which is visible on the network and the hostname, which is the thing you see in the terminal.

To change the network name you simply go to System Preferences->Sharing and change the Computer Name.

To change the hostname simply type

where your desired hostname is slackbook
Close the terminal and start it again and you should see the new name.