Obviously you fix the leak first. Depending on the amount of warpage in the ceiling,you can cut out the badly warped plaster and drywall over it or just drywall. Drywall is easy and relatively cheap. The finish will be the hard part. Patching cracked plaster has techniques which I am familiar with but I am in California and plaster cracks in earthquakes so I don't do it much.E-mail me for more info.