If the printer is AirPrint compatible just use AirPrint. The following lists compatible printer and gives other info.
Otherwise you will need an appropriate app. Some apps are printer/manufacturer specific and others are more general.
You may have to do the following to download an app
Starting when iOS 7 was released, Apple now allows downloading the last compatible version of some apps (iOS 4.2.1 and later only)
App Store: Downloading Older Versions of Apps on iOS - Apple Club
App Store: Install the latest compatible version of an app
You first have to download the non-compatible version on your computer. Then when you try to purchase the version on your iPod you will be offered a compatible version if one exists.