Before installing OS X 10.11, your need to make sure that you have at least OS X version 10.6.8 installed. Check this by clicking on the 'Apple' menu, and About This Mac.

Your Mac should show at least OS X Snow Leopard 10.6.8

If you don't have at least version 10.6.0, you'll need to purchase an OS X Snow Leopard DVD from Apple.