How do I run / install ROR with an outdated OS system?

I'm new to programming/development. I have an old Macbook Air. It's El Capitan OS X - 10.11.6.

I can't download the latest OS due to the age of my mac. I can't download the latest xcode due to the age of my OS.

Is there a guide to installing older versions of Rails/XCode/etc that are all compatible?

I've spent a few hours looking around but can't find any obvious solution. I've started trying to install some things through guides but there are quite a few error messages coming up.