When you import cds into iTunes it turns into a format called M4A. So to change it to MP3. Open iTunes and then go to Edit>Preferences>Advanced>Import.
Do the following:
Then that changes the encoder of importing cds. Then select all the songs you want to change to MP3 format and then right click and then there should be an option - Like this:
Then that changes the encoder of importing cds. Then select all the songs you want to change to MP3 format and then right click and then there should be an option - Like this: