Spellcasting Ability

Charisma is your spellcasting ability for your warlock spells, so you use your Charisma whenever a spell refers to your spellcasting ability. In addition, you use your Charisma modifier when setting the saving throw DC for a warlock spell you cast and when making an attack roll with one.

Spell save DC = 8 + your proficiency bonus + your Charisma modifier

Spell attack modifier = your proficiency bonus + your Charisma modifier