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.
A Stabilized creature doesn't make Death Saving Throws, even though it has 0 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. A stable creature that isn't healed regains 1 hit point after 1d4 hours.