Tiling shower walls how and where to start
If both corners are equally visible the best way is to start in the middle plumb on center. As you get to the corner measure and cut to fill. The leftover peice is the size of your starter for the next wall. Draw a new plumb line on the short wall according to off cut. This method does not always work out. If you end up with a tiny piece in the corner, offset your center line until visually appealing. 

If your tile is 16 wide it would work out like this

Your short starter peice on the short wall would be about 5-1/2 inches

Starting at one corner is much easier but real professionals will normally start on center.