Put ATF Type F fluid in the trans.
remove the drain plug under the motor and let it all drain out. Put the drain plug back in and remove the side plug, and the top plug BOTH. With a funnel or something, add ATF untill it dribbles out the hole in the side of the trans. them imediatly replace the side screw and then the top screw. Than your set!! If you dont have any fluid in your transmission now, and it has been doing it for long, you might have screwed up your transmission.