New software on old device
The last available version of iOS for the iPod Touch 3rd generation and iPad first generation is iOS 5.1.1. It is not possible to update your iOS device beyond this version.

You can download apps for older versions of iOS by adding the app to your purchase history using iTunes on your computer, since iTunes is version-agnostic. Once you've downloaded the app on iTunes, it will be in your purchase history and attempting to download it again on your device should prompt you to accept an older version of the app.