The turning point and the climax may be separate, though closely related, or simultaneous events. In The Magician's Nephew, the turning point and the climax are separated by moments. The turning point is when Digory faces off with the Witch, she with darkly stained lips, he with the apple in his pocket. The climax is when Digory realizes the Witch is tempting him to do something nefarious ("You needn't take the little girl back with you, you know") and defies her, effecting everyone's escape from her.

[T]he other things the Witch had been saying to [Digory] sounded false and hollow. And even in the midst of all his misery, his head suddenly cleared, and he said (in a different and much louder voice): "Look here; where do you come into all this? Why are you so precious fond of my Mother all of a sudden? What's it got to do with you? What's your game?" [...] "Up then," said Digory, heaving [Polly] on to Fledge's back and then scrambling up as quickly as he could. The horse spread its wings.

A turning point is defined as the moment when the conflict in the story reaches a crisis resulting in a turning point in the protagonist's actions. It is the crisis at which the protagonist and antagonist meet and challenge each other ("Why do you run from me? I mean you no harm. ... stop and listen to me now,..."). The turning point crisis occurs just before or simultaneously with the climax.

The climax is different from the turning point as it is the result of the crisis. The climax is the point at which the conflict is won (or lost) by the protagonist and at which the outcome of the conflict can be predicted: because Digory sees through the Witch's manipulations and deceits and escapes with Polly and Fledge, it can be predicted that he will return to Aslan with the apple and that Aslan's plans and rewards will go forward, defeating or stalling the antagonist.

[Digory] walked up to Aslan, handed him the apple and said: "I've brought you the apple you wanted, sir."

The turning point/climax of the book comes just before the resolution. The turning point is Digory's extended struggle with temptation (trying not to eat the fruit for which Aslan sent him). It is made worse by the fact that Jadis is there, tempting him. The climax follows soon after, and leads into the resolution: he goes back to Aslan, and is given the fruit to use to cure his mother.

It is a rather quiet and personal struggle after all the world-creating in the novel, but it works well.

What are the main events leading up to the climax of The Magician's Nephew?

Hmm. How far back do you want the chain of events followed?
Polly and Digory go to Charn.
Digory wakes Jadis.
Jadis goes with them to Earth, causing havoc in London.
Polly and Digory use the rings to take Jadis out of London, and accidentally bring others with them.
Rather than going back to Charn, they go to Narnia, where they are present for Aslan's creation of the world. This introduces evil to Narnia, and Digory must go retrieve the magic fruit to fight it; this leads directly to the climax, in which his mother is healed.

Certainly the Narnia/evil/fruit are directly connected; perhaps the others, too?

What is the denouement (the ending) of The Magician's Nephew?

It is interesting because the denouement, which wraps up everything in this story, actually sets the stage for the beginning of "The Lion, the Witch, and the Wardrobe."  "The Magician's Nephew" ends with Digory saving his mother by bringing her an apple from Narnia, which he managed to do by resisting the temptation to eat it himself.  Afterward he takes the apple core and buries it in the backyard.  It grows into a magnificent tree with the best apples anywhere in England.  Later, when the tree is destroyed by a storm, Digory has the wood made into a wardrobe, thus setting the stage for what many consider to be the first book in the series.

Check the links below for information on both denouement and the story itself.  Good luck!