Learning magic.
Magical abilities reveal themselves during childhood, and have to be learned and practised in order to use them properly.
Young witches join local witch classes, which usually small, and are attended once a week. They are taught the basic lore and how to use their magic properly. Other supernaturals are taught how to handle their abilities at home. Once they've finished high school they can follow a specialised training/school that prepares them for some job within the supernatural society, if they wish.
What magic can and can't do.
Magical abilities are increased by studying and training, like any other skill.
Magical abilities are amplified by a number of things. A spell can become more powerful when several witches work together to cast it. Working magic on a crossroads amplifies magic. The way in which you cast your spell can also amplify the magic.
Magical abilities are decreased by stress, strong emotions, mental illness, head injuries and substance abuse. Severe head injuries andmental health issues can make the magical abilities dissappear. Using your magical abilities costs you physical and mental energy. When depleated, you can experience symptoms like hunger, thirst, tiredness, headaces, blurry vision, dizziness and general weakness.
Some things have influence on magic, when they are incorporated into a spell. Things like herbs, crystals, symbols and objects, used for sympathetic magic. These things only have this working, if they are empowered with magic.
All magic users have their limits. There is no such thing as the ultimate powerful magic user, or "the one who will save us all", and there are no mages who are super powerful without practising.
The strenght of magical abilities varies from person to person.
Using too much magic can cause sickness, fainting and vision loss.
It is forbidden to use magic to harm or control another being. Death magic (in whatever form) is strictly forbidden.
Magic can't make more of something, predict precise future events, to fly and do time/instant travel.
Personal differences between magic users.
Not all magic users are equally powerful. Some may have very strong abilities, or may be able to do a wide variaty of spells, while others may have weak powers, or only be able to do a few spells. Kaylee, for example, can influence the way others perceive her, but she can't change her actual appearance or do any other form of illusion magic. She is, however, very good at the illusion magic she has, because she's been doing it since she was very little.
Most witches favor a certain type of magic, such as watermagic, spiritwork, enchantment. This does not mean they are able to learn everything there is to know about this type of magic. Ability is very personal. Raina, for example, has a talent for fire magic but does mostly active magic like shooting fireballs and creating fire. She can't scry in fire or control fire elementals.
Having a talent for one type of magic does not mean you can't do anything else. Being a police agent, Raina can also track magic and seal magical evidence. These spells have nothing to do with fire magic.
Every witch favors a certain way of conjuring/detecting magic, such as: through spells, gestures, sigils, rituals (mostly used in spiritwork), through the aid of spirits or with tools. A witch can make her magic more powerful if she gets better at her way of conjuring. For example: a spell casting witch can make certain spells more powerful if she sings them.