Special Effect (fog to ghost)

The most efficient workflow depends on what kind of footage and other assets you have. Generally, it is a lot easier to start with the image of the ghost then turn that into the fog, then pre-compose and time remap the comp so that the fog turns into the first frame of the ghost animation.

Tell us what kind of images you are working with, give us an idea of your experience level with AE, and let us know if you have any 3rd party plugins like Particular or Form. We can be a lot more help if we have some details.