I need help Installing OS X El Capitan

Sadly, Apple no longer offers a workable El Capitan installer off of the Apps Store if that is what you where trying to access. Recovery also is not going to work either as the certificate the recovery image files have expired as well and Apple has not repaired them.

Do you have access to a second Mac system which has El Capitan on it? You'll need to create a Bootable OS installer CD. Your MacBook doesn’t offer FireWire so you have no means to boot off of an external drive (only optical).

The other option is to take your HD out of your system and using a HD case connect the drive to the second Mac to install the OS

Here’s more on the certificate issue that you are facing; If you've got an old macOS install image, it will probably stop working today

Here’s how to alter the date from within the Date & Time control panel which you can access from the menu bar or via the control panel (System preferences) Here we are turning off the auto date & time setting.

Here’s how to change the setting using Terminal Changing system date from Terminal – OS X recovery. But if your system accesses the internet the Apple Time Server will correct your alteration to the year before you know it! So make sure you do this in the middle of a corn field miles from any WiFi network!