Do I need iPhone developer certificate and the app available on the app store in order to implement game center.
Is it easy to implement game center leader boards into your game and if so any tutorials on doing so. I have a score counter already ,but not sure how I would add it into game center.
Any good tutorials on implementing game center into cocos2d and if so may I see a link (yes I have seen some tutorials ,but I want to be recommended a good tutorial).
You need an iPhone developer account to test and implement game center. You don’t need to have an app on the App Store you can just test it on an unreleased project. I suggest you read the following tutorial, which explains everything about leaderboards pretty clearly:
Leaderboards Tutorial