At first you download and install the cocos2D library to your mac. The instructions for downloading and installations are available from here.
Now you can create cocos2D iphone application in your xcode simply by following the steps given below,

1. Start xcode and select cocos2d

       2. then name your project and push okey


   3. you can see your helloworld scene from the class folder now


   4. Now build and run.  Wow you created your first cocos2d app..


 Now the time for adding sprites, buttons etc and make a good looking iphone app.