You need a minimum of 25,000 to book a flight and that is on the skysaver. Additional seats are avaialbe for more miles.
The best way to see what is avaialbe is to go to Delta.com, log in with your skymiles account and go to the Skymiles areas. You will see a "use your miles" area. Click there and you can play around with making reservations.
I have had luck doing it one of two ways. Plan VERY far in advance (they accept reservations 330 days in advance) or last minute.