So, your world’s got magic. Great! Since you’re in charge, it’s up to you to dole out that mystical power to your characters. Time to decide who gets to participate.
A Matter of Study
Anyone with a keen mind and a good work ethic can learn the arcane arts. Maybe it’s something where a precocious student can figure out the basics on his own, but more commonly they’ll need to learn from someone who already knows. This can come in any number of forms: a one-on-one mentor, a university, reading old books, talking to spirits, or possession by demons.
There is a whole subgenre devoted to magical schools and instruction. Harry Potter, The Magicians, and The Kingkiller Chronicles all feature magic schools.
How readily one can find this sort of magical instruction? That gets into an issue I’ll discuss below: commonality of magic.
You’re either born with magical potential or you aren’t. Maybe there are rare circumstances (protagonists are suckers for unique circumstances) where someone else can gain the power, but generally it’s preordained. You can call it genetics, blood bonds, or divine heritage, but somehow that potential is passed down from one generation to the next.
This method lends itself to reveals of magical power that hint at secrets of heredity. When Joe Mundane suddenly conjures fire, you begin to suspect that those kindly farm folk who raised him might not have been his real parents.
Want to be a wizard? Well, you’re going to need a staff. Or a wand. Or an amulet or whatever other trinket channels mystical power in your world. If possession of the object changes hands, so does the associated power. This method doesn’t often stand alone, but is commonly associated with possessing more power. But the intermediary device can also serve as the sole means of working magic.
In a way, this method borders on the technological. Humans in the real world can’t fly without a plane or light fires without a match (or lighter, etc.). This is the magical equivalent, though the uses of magic may be more broad than single-function technological devices.
How Common is Magic?
A world where five people have any magic at all will have a much different feel than one where the waiter at a cafe will heat your tea with casual pyrotechnics. Decide whether you want magic in your world to feel like something rare and shrouded in mystery, so common that no one gives it a second thought, or somewhere in between.
The more common magic is to your world, the more magical the world will seem to readers. Conversely, a world with exceptionally rare magic won’t feel terribly different from one with no magic at all. However, the impact of a focus on magic in the story itself will scale inversely with how common it is.
Is Magic Desirable?
Are the people who have magical power envied, feared, or denigrated? This is entirely a matter of in-fiction cultural interpretation. The magic users themselves can influence the balance between obedience and persecution based on their skill, power, and attitude toward those without such power.
Also consider whether there are drawbacks to magic use in your world. Will a habitual user be harmed by accessing their power? Or maybe using magic draws the attention of beings from another dimension. Or it could just be exorbitantly expensive to purchase the materials needed for magical rituals. Give some thought to the kind of person who is willing to overcome these obstacles and still use their power.