Each weapon, spell, and harmful monster ability specifies the damage it deals. (96)
1. Roll the appropriate damage die or dice. The damage will be of a certain type.
2. For a melee or ranged weapon attack, add the appropriate ability modifier.
Your proficiency bonus is not added to damage. (64; 96)
Round down on the damage total, even if the fraction is one-half or greater. (PHB 7)
Damage is subtracted from the target's hit points. This has no effect on its capabilities until it drops to 0 hit points. In a melee attack, you can, if you wish, forgo killing a creature and instead knock it out.