Minecraft: How to use the /fill command.
/fill <x1> <y1> <z1> <x2> <y2> <z2> <TileName> [dataValue]
Where X1 Y1 Z1 are your first set of coordinates, then X2 Y2 Z2 are your second set... anything within these points will be replaced by given TileName.
If for instance you wanted to do Black Wool you would go to HERE for the block's dataValue.
/fill 0 0 0 0 0 0 wool 15
/fill 0 0 0 0 0 0 wool
would default to White Wool.