The first GameDev.tv course I took was on how to use Git and GitHub (Get Git Smart Course: Learn Git in Unity, SourceTree, GitHub). It gave me the foundations I needed to use Git, and later Plastic SCM (rebranded to Unity Version Control) as well. This knowledge has ended up being invaluable, and I now use it on a daily basis.
Through this course and additional learning I can now easily:
Commit
Push / Pull / Fetch
Branch
Tag
Rebase
Interactively Rebase
Create pull requests
Rewrite history when needed and edit my branches to look pretty
Edit
Merge
Cherry Pick
Fixup
Squash
etc.
I primarily use Fork, but have also used Sourcetree, Unity Version Control, and the command line. I use GitHub consistently for personal projects, browsing, and contributing to FOSS projects.
I've also learned various workflows such as Git Flow and GitHub Flow.