Upgrading early 2008 iMac?

You may also need hardware upgrades. Most off-the-shelf 2018 iMac configurations only shipped with 2GB RAM installed. Mountain Lion (10.8) was the last OS to play nice with 2GB RAM. In practice running El Capitan requires at least 4GB RAM, with 6GB RAM recommended.

Anyway, there are two methods to install El Capitan.

a) Find a Snow Leopard installer (white label, avoid model-specific grey-label recovery disks) and then upgrade from there.

b) Use another Mac to create a bootable USB installer. Create a bootable installer for macOS - Apple Support. This once and may still require using an El Capitan compatible Mac (Early 2016 or older).

Is there any particular legacy MacOS software you are trying to run? If you are just trying to get an old all-in-one computer connected to the internet you may have better results installing a medium/lightweight Linux distro such as Lubuntu or Fedora XFCE. If you create a Live USB/DVD you can test drive before installing to the internal disk.

Leopard is so old that it can't handle modern https security.

I would suggest using a modern computer to download the .DMG file containing the El Capitan installer onto a USB drive. This will not create a bootable installer, just a drive with a .DMG data file on it. Transfer the drive to the 2008 iMac and copy the .DMG file to it. Eject and disconnect the external drive. Then open (mount) the .DMG disk image and run the El Capitan installer inside.

How to download and install macOS - Apple Support

If you try this method and still get a message that you need 10.6.8 or later, you may need to see if you can upgrade to intermediate versions of macOS (Lion, Mountain Lion, or Yosemite) (in the same way) and then to El Capitan.

Apple once sold Snow Leopard on DVD but has not done so in a very long time. There is a Snow Leopard 10.6.8 combo updater at the Apple site, but it will only work if you already have some version of Snow Leopard installed.