Usage: message who what
If you are dead and a spirit hovering over your corpse, you can send a death tell to someone to get them to come rescue you. This sacrifices a piece of your spirit to send a message to the mortal plane (i.e. you lose some skill to accomplish this) so do not send too many.
In general you should make your message concise and include your name, directions to where you are (as accurate as possible), what killed you and the presence of any threats that might still be in the vicinity.