My top 3 recommended sources for learning about CI/CD and Git
DevOpsLaunchpad
This first one was created by GearSet, arguably the best tool/platform for Continuous Integration/Continuous Deployment for Salesforce.
Katacoda
This is O'Reilly's platform for learning, but it is only publicly available until June 15th! It lets you play with live interactive scenarios and sandboxes.
Trailhead - Git and GitHub basics
This is probably where you should start if you are entirely unfamiliar with Git/GitHub.
If you have any other favorite, please post in the comments!