A common point of confusion for new readers is that there are two valid orders: the (how the books were released) and the Chronological Order (the order of events in Narnia’s history).
The Magician's Nephew works best as a prequel. The mysteries of the wardrobe, the lamp-post, and the White Witch's origins are more powerful when you encounter them in Wardrobe first. Reading Nephew first removes the wonder and suspense.