You will need to scrape and sand to remove the loose, flaking and peeling paint. The prime will need to be a 100% acrylic primer so as to adhear to the plaster better as a regular latex primer which will not do anywhere near as good a job. This type of primer will often be called an undercoater, but the people at the paint store will be able to guide you in the right direction. It is your choice as to if you want to skim out the rough edges or not. The finish paint should be one made just for ceilings as they dry flatter and don't show up the rough areas as bad as a regular wall paint will.