Highlight the song in your iTunes and click "Advanced" on the upper menu of your player.
Click "Create AAC Version" in the "Advanced Menu" and allow a few seconds for the AAC version of your song to load directly below the original version. AAC is short for Advanced Audio Coding, which is the file format compatible with most cell phones.
Right-click on the new AAC song (if you intend to send it to your phone as a ringtone), and click "Get Info."
Click the "Options" tab and change the "Start Time" and "End Time" of your song to a clip of 30 seconds or less, as this is the length allowed for ringtones on most phones. Click "OK."
Navigate to a ringtone and media uploading site, such as Myxer or FunForMobile, and upload your song onto the site using the site's media uploader.
Enter your cell phone number and have the file sent to your phone as a text message. Open the file and assign it as a ringtone or save it to your media library.