Installing Xcode 7.x on El Capitan 10.11
The last Xcode version compatible with El Capitan is Xcode 8.2.1. If you can't install it from within Apple App Store, visit Apple Developer and download the app there. An Apple ID is required.

Xcode is provided as xip file (signed zip). You can simply unzip it by using Archives.app (i.e. with a double-click).