From the PHB, page 205.
SAVING THROWS
Many spells specify that a target can make a saving throw to avoid some or all of a spell's effects. The spell specifies the ability that the target uses for the save and what happens on a success or failure. The DC to resist one of your spells equals 8 + your spellcasting ability modifier + your proficiency bonus + any special modifiers.
ATTACK ROLLS
Some spells require the caster to make an attack roll to determine whether the spell effect hits the intended target. Your attack bonus with a spell attack equals your spellcasting ability modifier + your proficiency bonus. Most spells that require attack rolls involve ranged attacks. Remember that you have disadvantage on a ranged attack roll if you are within 5 feet of a hostile creature that can see you and that isn't incapacitated (see chapter 9).
So to use your example, you first have to manage a melee attack roll against the target's armour class in order to actually hit with the spell, and then the target gets to try their saving throw against your spellcaster DC in order to resist the spell.
In this case, the spellcasting ability is charisma, so the DC that the target has to make with its constitution save is 8 + your charisma modifier + your proficiency bonus. With 16 charisma and level 3, this means 8 + 3 +2 = 13. With 18 charisma, it would have been 14. At level 5, your proficiency modifier goes up to +3, but for now we're stuck at +2 proficiency modifier.
