From the Motorola Website:
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Follow the steps listed below to set up and use ringtones.

1. Set any of your songs as a phone ringtone:

From the music player library, navigate to a song you want to use as a phone ringtone
Touch & hold the song in the list
Touch Use as phone ringtone. The ringtone is now set as your default incoming call ringtone
2. Personalize your ringtones:

From the Applications menu touch Settings
Touch Sound & display
Touch Phone ringtone or Notification ringtone and select a ringtone
3. Customize the text message ringtone

From the Applications menu, touch Messaging
Press the Menu button
Touch Settings
Touch Select Ringtones

4. Assign a ringtone to a contact:

From the Applications menu touch Contacts
Scroll to the desired Contact
Press the menu key
Touch Options
Scroll down and touch Ringtone
Choose a ringtone from the list and touch OK
5. Download ringtones from Android market

From the Applications menu, touch Market
Touch search to search for ringtones