I will be installing ceramic tile (8X12) on a sheetrock ceiling above my tub.

Little tricky overhead. First, you have to make sure the drywall is well attached to the joists and preferably 16" centers. I would make sure to use Hardie backer or similar, well screwed to the joists too. I don't use thinset on ceilings, it doesn't have enough quick grab. I use Acryl Pro. Read the application thickness and trowel size for the tile. Do your layout and start the tile pressing them into the mastic with a twist. Some people tap on a padded 2X4 to make sure application is tight. You need to be very careful to make sure the tiles don't drop or loosen. Some people set 3-4 rows, use a 2X4 "T" bar to hold the tiles and wait a day or two before setting more rows. Is there a vent or light over the shower?

Does the tub also have a shower? Is this standard drywall and how thick is the sheet?