Before talking starting points, let's run over what "The Magician's Nephew" is about and how the book fits into the larger timeline of Narnia novels. Spoilers ahead!

The story of "The Magician's Nephew" takes place decades before the Pevensie children (Peter, Susan, Edmond, and Lucy, the heroes of "The Lion, the Witch, and The Wardrobe") are born, approximately at the turn of the century. We're talking about a Sherlock Holmes setting — in fact, that parallel is even made in the book. The narrative follows a pair of children, Digory Kirke and Polly Plummer, who live in London, where they stumble into a nasty adventure via the former's evil Uncle Andrew, who plays the part of the titular "Magician." To make a long story short, Uncle Andrew uses magic rings to send the children into a "world between the worlds" where they can enter into other worlds by wearing their rings and jumping into different pools of water that function as portals.

This leads the children into a dying world called Charn. They awaken an enchanted witch-queen named Jadis, who follows them first back to London, and then into an empty, uncreated world. There, they witness Aslan himself sing Narnia into existence — yes, this book features the creation of Narnia itself — and eventually, the wicked Jadis flees (though we know, of course, that she'll come back to become the White Witch). The children are sent home with a magical fruit that Digory uses to heal his sick mother. The super-powered piece of produce does the trick, the kids bury the pit, and eventually, Digory uses the magical tree to build — you guessed it — a wardrobe.