This is not an easy task to achieve but you can send up to 25 entries a day and you can enter on Apple's website.
Your credit can unfortunately only be used for digital content. So while you cant purchase any iPhones or iPads with your winning, you can spend the credit on iTunes, the App Store or the iBooks Store. The prize should keep your app needs under control for a very long time.
At the time of writing the counter is at the 24 billion range, so click the image above to head over to Apple's website and enter right now. You can enter the promotion by downloading an app from teh App Store or by filling the online enter form here.
Click here for the official rules