How to hide sidebar preview (on the right) in Finder on El Capitan
Solution 1:
This is called Preview.
Type Shift+Command+p to turn it either on or off.
Alternatively open Finder's View menu and select Hide Preview / Show Preview.
Solution 2:
The instructions provided by TechRaf only temporarily hide the preview panel. If you want to permanently hide it, from a Finder window, select View | Show view options, then un-tick 'Show icon preview', then click 'Use as defaults'. (Note that this will change all of your defaults to whatever settings are shown.)