Have a look at the following, each article or link has different key things required to make your final product. You eventually will be using a combination of geolocation, the compass/or the iphone's gyroscope data coming in.
Reading all the references combined and implementing them one by one in different projects will give you a solid start on how to then combine it all together to create your application. But first you need to get a solid understanding on how to manipulate the knowledge you will learn and how you can then apply it to create your project.
References:
A cool project from Ray Winderlech teach you how to use location gps coordinates in your application
The next two links show you how to grab gyroscope data to find out the pitch, yaw and rotation and find out the device current position in space.
Will teach you how to use the compass
Here's some more augmented reality stuff on overlaying stuff on the camera view