2. In the menu bar at the top of the iTunes window, click on Edit and select Preferences....
3. Click on the General tab and then click on the Import Settings button.
4. In the Import Settings window, select the MP3 Encoder from the Format drop-down menu.
5. Choose a bit rate from the Setting drop-down menu. The higher the bit rate, the better the sound quality but the larger the file size.
6. Click on the OK button to save your changes.
7. To compress a music file, select the file in iTunes and then click on File and select Convert and then Create MP3 Version.