Clean Install of OS X El Capitan
One thing that you can try if you need to give your Mac a fresh start or if you cannot solve a OS X El Capitan problem is to perform a clean install. This will wipe your Mac and install the new OS X 10.11 update on it just as if you bought it new at an Apple Store.
You'll need to use this guide to perform a clean install of OS X El Capitan on your Mac.
If you tried the OS X El Capitan beta and you need to upgrade, you don't need to go through that trouble, but you will want to use this guide to upgrade.