To calculate damage in DnD 5e, roll the appropriate damage die or dice and add any modifiers that apply to the attack, spell, or ability.
When attacking with a weapon, roll the weapon’s damage die and add your attack ability modifier, which is:
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Strength for melee weapons
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Dexterity for ranged weapons
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Strength or Dexterity (your choice) for finesse weapons
So, if you land an attack with a mace, the damage it deals = 1d6 + Strength modifier.
A spell will tell you which damage dice to roll for damage. Normally, you don’t add any modifiers to spell damage, unless a specific spell description tells you to.
If a spell deals damage to more than one creature at the same time, you only roll the damage once.
For example, if you roll 8d6 for a Fireball’s damage and the result is 30, the Fireball deals 30 damage to all creatures who fail their saving throw against the spell.
If a creature is vulnerable to a damage type (like fire or bludgeoning), double the damage that attack deals. If a creature is resistant to a damage type, cut the damage in half.
