Your Mac cannot be upgraded to El Capitan. Early and mid-2006 iMacs can be updated to Snow Leopard. Late 2006 models can be updated to Lion. Both are available in the Online Apple Store (enter the name in the Search field on any Apple webpage) but you have to install Snow Leopard before you can install Lion. Snow Leopard comes on physical install disks, Lion as a download code for the Mac App Store (installed with Snow Leopard).
This Wikepedia page lists the 2006 iMacs so you can tell which one is yours.
https://en.wikipedia.org/wiki/IMac_(Intel-based)#Polycarbonate_iMac