Here's some good info?

http://www.finaid.org/fafsa/headofhousehold.phtml

Check your state and/or county laws as you have to have a separate residence for X period of time before you can get the divorce; I don't know if for this form you have to have been in a separate residence for any period of time (but the above info says no state allows "separated" to mean cohabiting in same house)?