Paul The first book written and published was The Lion the Witch and the Wardrobe. The Magician's Nephew was the sixth to be written and published. The events in Magician's Nephew take place before the events in LWW, but they're written in such a way that reading LWW first makes more sense.
Jez Keighley This is a prequel, but it only makes sense if you read the Lion, the Witch etc first. Personally I would read this 2nd.