Your casting ability is the middle value of your three mental ability scores (usually). As an example if you have a 10 Int, 12 Wis, and 14 Cha, Wisdom is used for you spells Save DC's and other effects. If you have a 10 Int, 10 Wis, and 12 Cha, you can use Intelligence or Wisdom. If you have a 10 Int, 12 Wis, and 12 Cha, you can use Wisdom or Charisma.