Uh-oh. If you have to ask about this, there’s trouble. Luckily for you, the rules are pretty straight forward, how you got to this point was probably not so lucky.
Dropping to 0 Hit Points
When you drop to, or past, 0 hit points, you either die outright or fall unconscious.
Sorry, did you say “Instant Death”?
Massive damage can kill you instantly. When damage reduces you to zero hit points and there is damage remaining, you die if the remaining damage equals or exceeds your hit point maximum.
For example, a cleric with a maximum of 12 hit points currently has 6 hit points. If she takes 18 damage from an attack, she is reduced to zero hit points, but 12 damage remains. Because the remaining damage equals her hit point maximum, the cleric dies.
Basically, they did away with negative hit points (back in the day it used to go to -10 and you were dead, dead) and anything below zero is considered zero hit points. UNLESS it’s way below zero, to the point of the negative of your hit point maximum. If you get damaged to that point your toast. pffft. Gone.
If damage reduces you to 0 hit points and beyond (but not waaay beyond), you are at zero hit points and you fall unconscious. This unconsciousness ends if you regain any hit points.
Death Saving Throws
Whenever you start your turn with 0 hit points (there isn’t negative hit points anymore, unless it kills you outright, you’re at zero), you must make a special saving throw, called a death saving throw, to determine whether you creep closer to death or hang onto life. Unlike other saving throws, this one isn’t tied to any ability score. You are in the hands of fate now, aided only by spells and features that improve your chances of succeeding on a saving throw.
Roll a d20. If the roll is 10 or higher, you succeed. Otherwise, you fail. Success or failure has no effect by itself. On your third success, you become stable (see below). On your third failure, you die. The successes and failures don’t need to be consecutive; keep track of both until you collect three of a kind. The number of both is reset to zero when you regain any hit points or become stable.
Rolling 1 or 20. When you make a death saving throw and roll a 1 on the d20, it counts as two failures (sucks big time to be you, throw away that dice if this happens, they obviously hate you). If you roll a 20 on the d20, you regain 1 hit point (buy a display case for that die).
Damage at 0 Hit Points. Ugh, some baddies just like to hit you when you’re down. If you take any damage while you have zero hit points, you suffer an automatic death saving throw failure. Unconscious creatures are also easier to hit, they roll attacks with advantage. If the damage is from a critical hit, you suffer two death save failures instead. Ouch. And you still have to make your death saving throw for the round. Things can go permanently bad very quickly if no one’s watching over your unconscious body. If the damage equals or exceeds your hit point maximum, you suffer instant death otherwise it doesn’t really matter. For example, you’re not instant killing an unconscious 20th level uber-barbarian with your dagger, but you can speed them along the path to meet their gods with it.
Stabilizing a Creature
The best way to save a creature with 0 hit points is to heal it. If healing is unavailable, the creature can at least be stabilized so that it isn’t killed by a failed death saving throw.
You can use your action to administer first aid to an unconscious creature and attempt to stabilize it, which requires a successful DC 10 Wisdom (Medicine) check. Remember, two people doing a task gives advantage, so help your buddies help your buddy. And it’d be nice if someone had the medicine proficiency to give a bonus to that role. You do have a healer in the party, correct?
A stable creature doesn’t make death saving throws, even though it has zero hit points, but it does remain unconscious. The creature stops being stable, and must start making death saving throws again if it takes any damage but the good news is that it’s a new session of death saving throws.
A stable creature that isn’t healed regains 1 hit point after 1d4 hours. At which point a short or long rest is probably in order to recover more hit points.
Monsters and Death
Most DMs have a monster die the instant it drops to 0 hit points, rather than having it fall unconscious and make death saving throws.
Mighty villains and special nonplayer characters are common exceptions; theDM might have them fall unconscious and follow the same rules as player characters. DM’s choice here for best storytelling effect. Maybe that left-for-dead unconscious baddy dodged death and comes back for revenge?