From the Friends list, touch the Menu Key > Settings
.
Checkmark Invitation notifications to receive a notification in the Status Bar when friend invitation(s) are received.
Touch Notifications in the Text chat settings and Voice & video chat settings sections to receive a notification in the Status Bar when a new chat message arrives.
Touch Ringtone in the Text chat settings and Voice & video chat settings sections to open a list of the ringtones that will ring to notify you of a new chat.
Touch Vibrate in the Text chat settings and Voice & video chat settings sections to open a list of the ringtones that will ring to notify you of a new chat.