You can change the kind of files that iTunes creates when you rip CDs and add them to your music library. Every file type has its own strengths and weaknesses – some have higher-quality sound, create smaller files. To do this, you need to change your iTunes import settings.
ITunes defaults to using AAC, encoded at 256 Kbps, aka iTunes Plus (the higher the Kbps-–kilobits per second--the better the sound quality). You might want to encode at a higher (or lower) rate or change to creating MP3 files.
To change these settings, begin by opening the iTunes Preferences window:
On a Mac, go to the iTunes menu and choose Preferences.
On Windows, go to the Edit menu and choose Preferences.