'The act of casting magic is important as well. Generally, just willing it to occur is enough for most spells. Some spells require verbal chants, but those are highly complex processes that you do not have to worry about right now. Anyway, casting magic requires Mana. All beings are filled with Mana, and the size of something frequently described as the "Mana Body" determines how much Mana you have. Casting a spell uses up some of that Mana. Over time, the Mana will replenish. The rate at which it refills depends only on where you are; certain locations have more Mana in the air than others, and certain materials, like Mana Crystals or Mana Fluid, release Mana naturally, allowing you to absorb it. It is difficult to quantify Mana, so you must depend on your own judgement to determine how much magic you can use.
