If you roll a 20 on the d20 for an attack roll, you score a Critical Hit, and the attack hits regardless of any modifiers or the target’s AC. A Critical Hit lets you roll extra dice for the attack’s damage against the target. Roll all of the attack’s damage dice twice and add them together. Then add any relevant modifiers.
Damage at 0 Hit Points. If you take any damage while you have 0 Hit Points, you suffer a Death Saving Throw failure. If the damage is from a Critical Hit, you suffer two failures instead.
Automatic Critical Hits (Paralyzed and Unconscious conditions). Any attack roll that hits you is a Critical Hit if the attacker is within 5 feet of you.
RollQuest ships the D&D 2024 SRD as a content pack, on a 5e character sheet that does the maths - roll from the sheet and let the numbers work themselves out. Free to start.