As a first read on the topic you might want to start from this 10 minutes read article .
A great set of tutorials on Git from Atlassian (maker of the popular version control system BitBucket) are available here and they are also providing a nice short and quick list of common Git commands here. Another nice cheat sheet of common Git commands is also provided by GitHub here .
A really great learning tool which I definitely suggest is the interactive Git cheatsheet available here .
Also to practice your Git commands knowledge you might want to try the online emulator at https://learngitbranching.js.org/.