It seems like you're asking from two things:
1. How do I write about my character before and as he gets his magic?
and
2. How do I introduce this world?
I have my answers for those questions:
The "origin" of a character can best be summed up by the modern source of the trend, the super hero comic. Every Super hero has an origin story that can be summed up in a few sentences. In film/literary theory this can be called the Premise or the Hook and it sums up your story in a sentence or two. But more often than not, the Origin story of a character is the Initiating Event(IE). A character's life begins in balance and then something throws that life out of balance and your character strives put his/her life back into balance. The IE is the first drop in the bucket, the first ripple on the pond, the first thing out of place in your character's world. "My Main Character was normal until..."
How do you introduce this world you have created? There is no right answer to that, I just know how I approach it because it has worked for me thus far. I like to think of a ficitional world, any fictional world from the Dresdenverse to Eberron to the place of Fairy Tales like something I'm not good at, a Sport. Explaining your world is like explaining to someone Hockey or Football or Baseball.
It all revolves around The Playing Field, The Players and The Rules of The Game.
The Playing Field of your story is your scope. In Baseball, there is the stadium and the bases, there is nothing outside of it for the players. In the Dresdenverse, the Playing field is the whole world but for Harry, it's just Chicago.
The Playing Field is your story's geography; this is what it looks like, this is where everything is, this where the story is going to be told.
The Players are the forces at work on the field. The positions, basically. In Baseball you have Batter, basemen, the pitcher, umpire etc. In literature you have your good guys and your bad guys, your allies and your enemies or more specifically, your protagonist who pursues his goal(s) and the antagonist who stands in the way of that goal.
The Rules of the Game are the most important part of the world to explain. Every story has to do this. It represents how the game is played and what is at stake. If you can explain the rules of Hockey, Baseball or Football to someone, you can explain a fantastic place easy.
The Rules of the game of a story include basic rules like gravity. "What goes up must come down." Injury "When the MC gets punched he bruises, if cut he bleeds." The Rules of the game include how the game is played and what is at stake.
For example, Harry Dresden starts in SFront by explaining what magic is and how a wizard is able to use it. At his first crime scene, he shows his Murphionic field , which is a rule. He explains how magic could have done the first crime in Storm Front and when he finally uses magic explains its rules for use. WHen he meets Marcone, he explains about a Soulgaze and its rules, etc.
The best place to observe Rules of The Game in action is actually movies and scifi television. A rule of the game in those mediums are Chekov's guns so you will always see them pay off.
I hope this helps you,
