Wondering where to even begin with the Chronicles of Narnia is a classic problem, but it's honestly not as complicated as some folks make it. The most straightforward approach is the original publication order, starting with 'The Lion, the Witch and the Wardrobe'. This is how most of the world first met Narnia, and the story introduces the world and the Pevensie children in a wonderfully organic way. It's just a solid entry point.
Some people passionately argue for the chronological order, beginning with 'The Magician's Nephew', which is technically the creation story of Narnia. I tried that once, and while it's interesting for a re-read, starting there robs you of the mystery and discovery you get from Lucy stepping into the wardrobe. You're told things upfront that were originally meant to be delightful reveals later. For a first-time reader, publication order preserves the magic the author intended. After the first book, you'd go to 'Prince Caspian', then 'The Voyage of the Dawn Treader', 'The Silver Chair', 'The Horse and His Boy', 'The Magician's Nephew', and finally 'The Last Battle'.