How to implement leaderboard with social api

Hey Everyone !
I am new in unity3d. I am in a deep problem as I want to implement a score leader in unity3d. I tried to use scripting reference but I could not find a way. Do I need to register my app to somewhere ?
Your help would be appreciated.
Thanks in anticipation.

there is a need to make a test app from apple development site plus we need to go through all the itune process of uploading the app…