How to use rmbp as a desktop

A bit more info about what you want to know would be great but in order to use the rMBP as a desktop with the lid closed you need to have the power adapter connected, otherwise you'll just send it to sleep.

 

http://support.apple.com/kb/HT3131

You need to have the notebook connected to power and an external mouse/keyboard to let you use it with the lid closed.

 

Clamshell mode:
http://support.apple.com/kb/HT3131

In a nutshell:
From your laptop, connect your external display, along with an external keyboard/mouse. Close the lid on the laptop. All goes to sleep.
Press a key on the external keyboard to wake up your Mac. Only the external screen will light up. And, the internal screen is ignored while the lid is closed.
As andyp350 pointed out, you need your external power adapter connected.
It's no longer portable while you use in clamshell mode (but you are using it as a desktop, right?)

 

my rMBP sits in a BookArc connected via TB to an ATD. External keyboard, drives, and Ethernet all connect to ATD.

 

If all else fails, you can try an app called "InsomniaX" (link below). I've used this before but it's been awhile.

http://semaja2.net/projects/insomniaxinfo/