MAC - Format and set up a clean install of OS X

- Save your data on your extern disk
- Download the last OS X version through the App Store
- Format a USB key through the Terminal by command: sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/Sans\ titre --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app --nointeraction
- Restart your computer and press the ALT button
- Optional: you may need to format your disk if you set up a new SSD hard disk for example (follow the steps though the Utilities Tools)
- Install your clean OS X