You have access to a magic system called the Script. This allows you to gain experience through living life, upon reaching an increasing threshold, you level up. Initially this system has 4 stats and a Class, Strength, Vitality, Willpower, and Focus. Strength is directly proportional to your max Health, while your Vitality is tied to your Health regeneration. Willpower and Focus are the magic versions, reflecting max Mana and Regen respectively. You are to receive points from leveling up and performing incredible feats. These points allow you to upgrade your stats and buy spells.
Even without putting points directly into your Willpower, you are able to increase the amount of magic you produce. Mana Production is affected by how well a person knows magic, how much magic they do, and the mana itself. These growths are based on your lifes feats, saving an entire city of innocents could net you a sizable increase, being an important teacher of magic would give you a much larger boost, leading a country would net you a robust boost.
Classes allow you to specialize in certain magics, you may design your own Class with enough skill. Each Class initially has five slots for bonuses that improve you tremendously, a single bonus would allow you to double your health, triple your regen, or even be something more esoteric such as an Affinity towards an Element. To get more of these bonuses you must complete various Quests or perform on the level of making an entirely new branch of magic.
However you should remain aware that the Script itself is a crutch, not every spell is purchasable, Time Magic for example requires the completion of an incredibly lengthy Quests in order to even gain the right to the most basic spells. In order to achieve some of the rarer spells you must work to craft them yourself, casting them out into the world upon the formation of the spell. If you are unsatisfied with the result of a combination, you may rip the spell apart inside your spirit, giving yourself another attempt at greatness.