Answer: nail a perfectly straight wooden strip and tile from the corner of the wall.
When laying tiles on a wall, you cannot start tiling directly from the floor, because a floor is never completely straight, even if it is not visible to the naked eye. However, you must lay your tiles along a perpendicular axis so that the entire structure is straight, you have no choice. To do this, you will have to tile from a horizontal cleat fixed at the top of the wall, not at the bottom. Choose how high you want to tile, and nail a long, perfectly straight wooden rod a spirit level. Then start tiling from the corner of the wall, taking into account the joints. Once the first row of top tiles is successful, the rest should follow without problem.