If you're picking up the Guardians of Ga'Hoole for the first time, start right at the beginning with 'The Capture'. Some people might suggest skipping around, but this series builds its world and characters really methodically across the first few books. You get introduced to Soren, Gylfie, Twilight, and Digger in that first one, and their founding of the Great Ga'Hoole Tree is the entire spine of the early arc.
After 'The Capture', you just go in numerical order through 'The Journey', 'The Rescue', and so on, all the way through to the fifteenth book, 'The War of the Ember'. That's the main story. There are also a few supplemental 'Guide' books that came out later, like 'A Guide to the Great Tree' and 'The Lost Tales of Ga'Hoole'. Those are full of fun lore but are best read after you finish the core series, as they contain spoilers and assume you know the characters.
Honestly, the biggest point of confusion I've seen is with the movie adaptation. The film 'Legend of the Guardians' blends events from several of the early books, so if you saw that first, the book order might seem a bit off. Just stick with the publication order—it never steers you wrong. The characters' growth from kidnapped owlets to true guardians is what makes the series work, and you'd miss that gradual development if you jumped ahead.