When you score a critical hit, you get to 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 as normal.
If you roll a natural 1 on your attack then it will be a critical failure. Bad things _might_ happen as befitting the current scenario as decided by the DM, or nothing may happen at all.
Examples: If you are using a flimsy or damaged weapon then it might break. A standard sword might get a slight notch that would need to be honed out during the next rest period; an arrow might miss and shatter against a wall or tree. You might slip or become unbalanced which provokes an attack of opportunity.
This same rule also applies to NPCs.
Drinking a potion can count as either a bonus action, or a movement action, so long as the potion is kept within easy reach such as in a belt pouch or a pocket. If it needs to be dug out of a satchel or backpack mixed with other gear then it still counts as an action. The location of the potion will need to be clearly stated otherwise it is an action.
Giving a potion to an ally also counts as a bonus action, however administering a potion to an ally counts as an action.
Drinking a potion will provoke an attack of opportunity if you are within a creatures threat range unless you take the disengage action.
Sleeping in Armor
All players in medium or heavy armor must remove their armor in order to use any hit dice to recover hit points. Players must remove their armor for the duration of at least 30 minutes to use their hit dice to recover after a short rest. A player must remove their armor for at least 4 hours to recover their hit points and hit dice during a long rest.
Bonus Action as an Action
If you have already used up your Bonus Action, you can still use another Bonus Action as an Action instead. So if for example you cast shillelagh as a bonus action, you can cast Healing Word as your Action even though it is a Bonus Action spell and you have already used up your Bonus Action. However the rules on casting two spells in one round still apply and you can’t cast more than one spell in a given round unless one of them is a cantrip.