Lap sealant can be bought at an RV store. It is a type of caulking that comes in a tube that you use with a culking gun.
Look for bad sections in a seam or around a vent, air conditioner, etc. If the ceiling is badly damaged on the inside you will want to remove this to avoid mold formation. This may also help with finding the leak. You may have to have an assistant either man the garden hose or watch for thr leak.
Once you find the leak all old caulk, etc., will have to be removed down to bare metal. If the metal is damaged it will need to be repaired or patched. You can post pictures here so advice can be given.
Main thing is to be sure you have found the source of the leak. If you find the source you can do a temporary repair to confirm the source, then do a permanent repair.