Find the middle of the wall, draw a plumb vertical line, level works nice for this

, now take a tile and place on the line and go iut to the end walls and see what size cut you get, if a small cut, start the tile with the line in the middle of the tile, this will give you even cuts at both ends, now start with a level line on the first course of tile, do the same, keep placing one and marking it out with grout ljoint space up to the ceiling and see what you get, ajust what's needed, now use modified thinset and spacers and go slow, it should work out.
Leave out the bottom row if need be and use a batter board to support the tile where you start, then when finished, remove the board and set the bottom row.
Got it?