A good place to start is Crutchfield. Enter your truck info and the site tells you what fits. Crutchfield is usually more expensive but you get lifetime tech support, instructions and usually they include other needed parts for installs included in the price. Whether you buy from Crtuch or not, it is a good starting point.
2011 F-150 XLT SCrew - Blue w/ Silver Ingot accent, 5.0L, American Racing AR708, MPT 87 Performance Street Tune, Advent/Polk/Kicker audio.