An outstanding series of tutorials has been posted on how to use Box2D with Cocos2D, and creating a simple breakout game using these technologies.
The tutorials are from Ray Wenderlich, the creator of the Space Game Starter Kit and co-author of the Learning Cocos2D book, and the first thing that you will want to do is check out Intro to Box2D with Cocos2D Tutorial: Bouncing Balls. In this tutorial you’ll learn how to create a Box2D project, and how to add physical characteristics to 3D objects with Box2D.
Next you’ll want to check out How To Create A Simple Breakout Game with Box2D and Cocos2D Tutorial: Part 1/2, in this tutorial you’ll use what you learned in the first tutorial and in addition turn the ball into an ever bouncing ball, and add a paddle that you can move across the screen along with how to set up collisions between the two.
Finally, go through How To Create A Simple Breakout Game with Box2D and Cocos2D Tutorial: Part 2/2, and you’ll learn how to place blocks for the ball to hit, and how to set up collisions between the objects and finally an end of game screen.
[Source: Ray Wenderlich]