Magical Disciplines are highly specialized knowledge about a certain type of magic. Easy or basic spells usually don't require such a knowledge.
Now how can they be attained? The easiest way is to get yourself a Grimoire (which can be created with the create grimoire spell) and begin to explore the world. Disciplines are usually hidden in mystic places or are rewards for dangerous quests.
If you found one, you'll automatically write it into your grimoire. By reading your grimoire you can find out which disciplines you already have.