The first book, The Lion, the Witch, and the Wardrobe was published in 1950.
The remaining books were published later. The series should be read in the order of publication:
The Lion, the Witch, and the Wardrobe
Prince Caspian
The Voyage of the Dawn Treader
The Silver Chair
The Horse and His Boy
The Magician's Nephew
The Last Battle
Some publishers rearrange this order to put them in "chronologial order", putting The Magician's Nephew in the beginning, and The Horse and His Boy third. This robs the series of a lot, as many things that are mysteries and secrets in the earlier books are revealed in The Magician's Nephew.