As a lot of kids did, I read the Lion, the Witch and the Wardrobe first. You'd be surprised at how many people don't know that this is the first book of Narnia, not LWW. I'm going to assume everyone knows the LWW story and say that if you read this after it, it clears up a lot of confusion, like who the Witch is and where she comes from, where the Wardrobe comes from and why on Earth there's a solitary lamppost.
We meet Diggory, who is living with his Aunt and Uncle due to his mother being sick, Polly, who meets Diggory in the Garden and they decide to explore the attic together, Uncle Andrew, a very unpleasant character and of course, Aslan himself. Even though this is an old book (technically a classic), it's never boring and isn't written in a confusing way. I'd recommend this to any fantasy lovers.