Fill the gap between the tile and the tub with caulk.
How to install ceramic wall tile around a bathtub.
Apply it to the wall with the bottom edge about 1 4 in.
Above the lip of the tub.
Follow the order shown in the diagram to snap the rest of the lines.
I cover some of the specifics of how to layout and set up a tub to install tile around it.
Then apply a generous amount of thin set to the bottom bathtub tile and position it 1 8 in.
The best approach when installing cement board or other tile backer around a tub is to keep the cement board out of the lip area.
This video will show you how to set tile around a tub.
If the tub is level start with a full tile at its rim.
Check if you need to adjust this mark to avoid laying slivers of tile at the top and bottom.
When tiling around a tub mark the first layout line at the vertical edge of the tile next to the tub.
For a tub surround only install the backerboard and tile 12 inches above the tub.