You will receive one training point every 4 levels till 20. After level 20, you will receive one training point every 5 levels. You can also receive training points through certain quests. These training points can be used to learn spells from outside your school of focus. In order to learn higher level spells of a school, you have to first learn the lower level spells.
If you make a mistake, you can sell back your Spells for Training Points with Mr Lincoln in Golem Court. This transaction costs Crowns.