To use custom ringtones for your phone ringtone, your alarm or your text message notification, simply copy an MP3 file to one of the following folders:
If they don't already exist, then create a new folder.
The file will then show up when you want to change your ringtones.
For Ringtones go to Settings - Sound - Phone ringtone
For Text Messages go to - Settings - Sound - Notification ringtone
For Alarm, set the sound when you add a new alarm