It should really not be the responsibly of kicad to explain either git or github. There are enough good tutorials out there already for that.
- https://www.atlassian.com/git
- https://guides.github.com/
- this 1:20 hour video gives a good explanation what git is and how the basics work.
Github simply hosts the git server and provides a nice web interface. (There are plans to transfer all of the kicad resources to a gitlab server on a cern server. But that will take some time.)
The stuff about creating symbols and footprints is well documented in the official kicad documentation and we have two additional tutorials here on the forum. (4 if you count the stuff about making new libraries)
- Tutorial: How to make a symbol (KiCad v5.1.x)
- Tutorial: How to make a footprint in KiCad 5.1.x (From scratch)?
The library management also has a tutorial here on the forum. This one even explains how a custom setup connected directly to github could look like: Library management in KiCad version 5 (Focused on how to get github changes into kicad. The other way round is not explained there.)