Whenever you hit somebody successfully with a weapon, this does one point of damage.
You do not need to call anything. You may call SINGLE if you think the target has not noticed, or may miss the blow.
The other damage calls are ZERO, DOUBLE, TRIPLE, QUAD, QUINT and DEC.
These do 0,2,3,4,5 and 10 points of damage respectively.
If an effect call says that you 'cannot attack', then, if you do hit someone, you must call ZERO to let them know that they should not take damage from your attack.