As for magic it is like most chronic illnesses, you are born with it but do not know until it has a reason to flare up. Again like a disease, it is random. You do not get to chose if you are born with it and someone who has had a long line of magical blood might find that their next offspring lacks the ability. Not just anyone can practice magic, you have to have a certain gene to do it. It is treated like a birth defect though and when children discover that they are magicians they are usually abandoned.
Most wizards and witches have one prime ability that they spend years honing and several, smaller ones that they practice on the side. For example if you're born with the ability to manipulate elements your prime power could be fire and on the side you practice wind and earth (you wouldn't be able to practice water though because it clashes with your prime). Anything that changes reality is not allowed, there are certain laws, a Codex they call it, as to what witches and wizards can and cannot do. I'll get more into that later.
Abilities like immortality are not allowed as it's not an ability, it's a cop out. Witches and Wizards don't have normal life spans though, once they hit twenty their aging basically stops so they can live until they are killed.
As for the introduction that was several hundred years ago, it's a story passed down by word of mouth to warn young magicians how cruel humans can be and that they must always trust the Guardians of the Forest. Children living in the kingdom (called Migistrate) would never had heard this because it is written in one of the many illegal books. Anything that has to do with magic/alchemy/sorcery/demonology has been outlawed and burned. Therefore you don't need to relate to what was written there. I bestow upon you the creative license to do whatever fits the guidelines.
Now, onto the Codex. It is a very large book containing the rules of what a Witch and Wizard can and cannot do. The old leathery edition will appear when someone discovers their abilities. The five most important rules are as follows;
I. No Reviving the Dead
II. Magic that changes reality is strictly forbidden.
III. If your abilities include going into the future or past, you are forbidden from changing anything. Observation is allowed, but using any information you currently have and going to warn your future/past self is frowned upon.
IV. Healing magic is allowed, but one must never use healing spells or potions on a body that is not accustomed to magic. The results may be tragic.
V. Under no circumstances is one allowed to kill any of the Guardian spirits, or spirits of the forest.
