How do I transition between tile and drywall in a tub surround?
A usual method is to go down the edge of the wall with a surface bullnose, showing a "knife edge" at the corner.  This means the tile rolls down to meet the corner.

Another method, if your bullnose has finished bottoms, is to overlap the edge and go "around the corner", having the bullnose round into the outside wall (the wall that has the striped wallpaper) The bullnose will hide the tile edge, putting that grout line facing the side of the entrance.

This photo shows both methods: 
the left with a flat-on-wall knife edge and
the right shows a wrap-around-the corner



You can use the edge profile pictured, in place of surface bullnose.  You would adjust your tile to end short of the corner (equal to the edge thickness of the profile.  



In all cases, grout would be filled into the corner/wall junction.