How to reinstall OS X El Capitan on a 2008 Mac?
i have a a 2008 model
[Re-Titled By Moderator]
iMac 21.5″, OS X 10.11
You will need its original, grey, System Install DVD. Boot from it, erase the startup disk, install OS X (Leopard, probably, but without knowing your Mac's exact model designation we can only guess), and upgrade.