I'm guessing you don't want to tear down the walls and frame everything up square.
That being the case, you're going to have to cut the last row of tiles on each side of the room. You said you found the center of the room. I assume you snapped a line between the two marks and used that as your starting point. Lay out your tiles dry so that the outside row of tiles on each side of the room when cut are about the same size. In other words, you don't want a 2" cut on one side and a 9" cut on the other side. You may have to adjust your center line left or right. As far as the tiles being straight on one wall, good luck. You'll find few rooms that are completely square. Its not as noticeable if you're using all the same color tile when the walls are out of square as it would be if you're doing a checkerboard pattern with two different colors.
How far out of square are the walls?