Welcome to Android Central! An Auto Rotate toggle is often in the Quick Settings--swipe down from the top with two fingers. If you don't see it there, then go to Settings>Accessibility and turn off Auto Rotate there.
VidJunky is correct--not many stock launchers allow for rotation to landscape on a phone. The Google Now Launcher does it, and so does Nova. Are you using the stock Sony launcher, or an alternate launcher?