This article explains how to convert iTunes songs to MP3. Instructions apply to iTunes 12 for Windows and Mac, but the process should be similar in older versions.
Convert iTunes Music to MP3 Files – EASY
Follow these steps to convert iTunes songs to MP3 files for use on other types of MP3 players (they still work on Apple devices too).
iTunes' built-in audio converter allows you to manage the conversion settings, such as the type of files you want to create and the audio quality you want.
To change the settings for this task on your Mac, go to iTunes > Preferences > General > Import Settings > select MP3 Encoder.