When you take the Attack action, you can replace one of your attacks with throwing a flask of Holy Water. Target one creature you can see within 20 feet of yourself. The target must succeed on a Dexterity saving throw (DC 8 plus your Dexterity modifier and Proficiency Bonus) or take 2d8 Radiant damage if it is a Fiend or an Undead.
RollQuest ships the D&D 2024 SRD as a content pack, on a 5e character sheet that does the maths - roll from the sheet and let the numbers work themselves out. Free to start.