how can i get excel on a el capitan

You don't. Microsoft only supports the current Microsoft 365 applications on (currently) Mojave, Catalina, and Big Sur. The license servers for Office 2008/2011 for Mac have been disabled, so you cannot provide a Microsoft key to use them.

The free LibreOffice 7.1.5 Suite has a Calc application which is intended to be a clone of Excel. I would use that as it is supported (presently) on macOS Yosemite (10.10) and later. There is a Calc user guide that may come in handy too.