If you're a new developer - I would say this is going to be challenging for you.
But off the top of my head:
- Make sure to use iOS7 APIs only for iOS7 - so no NSURLSession under iOS 6 for example.
- You will need to design two UIs (One for iOS6 feel and one for iOS7 look)
- Autolayout should take care of most of the UI layouts for different size screens
- Since you're supporting the iPhone 4 - run the app on the actual device to make sure you don't suffer performance issues.
Suggestion though: Since you're supporting iPhone 4 and up - why not drop support for iOS6?