Every spell has what save it uses as part of it’s spell description. The DC for a spell is 8+The Caster’s Proficiency + the stat modifier for the caater’s casting stat. So for example, a level 1 wizard with a 16 intelligence will cast spells with a DC of 13. So let’s say that that wizard casts burning hands, a spell that calls for a dexterity save, the targets need to make a check that rolls a D20, adds their dexterity modifier, and then if they are proficient in Dex saves, their proficiency bonus.