You should contact an agent immediately who is familiar with short sales. He/she should ask if you have applied for a loan modification. Although these are hard to get, sometimes they are successful. If your situation was temporary and you can see yourself as able to get caught up, you may be successful. If not and your if you are needing to get out from under, you need to contact an agent asap.
Interview several agents until you are comfortable with that agent. Be prepared to provide any and all documents requested of you, such as last two years tax returns, past and current check stubs etc . You will need to be prepared fill out an financial form and a hardship letter explaining to the bank why you are requesting them to accept less than what you owe. Your agent will also need you to sign a form that gives him/her permission to discuss your account on your behalf.
You should be trying to sell already, unless you don't mind having a foreclosure on your record.