Okay, it works like this: If you're a cleric, the Cleric class talks about this under proficiencies - "Trained in divine spell rolls and DCs and in attack rolls for divine spells"
So you'll be trained at level one, which means you'd get the class level bonus of +1. You'll also get the key ability bonus. If you've got a Wisdom of 16, you'll have the following Spellcasting DC:
10 (base) +1 (trained, level 1) +3 (WIS bonus) = 15
As you go up in levels, your spellcasting becomes more powerful, and the DC increases by your level.
