Your question is the equivalent of "how long is a piece of string"
Need more infomation to give you an answer.
*Why are you painting? change of colour/ problem with old paint?
*what type of material is the ceiling/type of paint on it?
*what room is it? a kitchen/bathroom or other
*what type of paint were you going to put on it?
They all may affect the answer you want.
But normally everything being Ok - wash with sugar soap, dry and apply water based paint-ceiling paint.