/setworldspawn [x] [y] [z] Sets the world's spawn to the specified coordinates. If no coordinates are specified, the world spawn will be set to the executor's current location. x, y, and z must be within the range -30,000,000 to 30,000,000 (inclusive). x, y, and z are not optional in command blocks.
For a complete list of SinglePlayerVanilla Minecraft Commands, go here.