If you haven't erased your hard drive, reboot normally, go to the App Store, and download El Capitan using your Apple ID. You can try to download it at the link below. If it won't let you, then your only options are to upgrade to Sierra/High Sierra/Mojave or find an older computer where you can download it using your Apple ID.
El Capitan
How to upgrade to macOS Mojave - Apple Support
How to get macOS High Sierra
Sierra download
You can make a bootable USB stick to install using these free programs which will do all the work for you. Boot off the USB stick and reinstall the OS.
Bootable USB Flash Drive – Diskmaker X Disk Creator or use Apple’s method Create a bootable installer for OS