Here are a few resources that might be useful:
- Git and Github in Plain English, super nice to understand the concepts: https://blog.red-badger.com/2016/11/29/gitgithub-in-plain-english
- a quick overview of git: https://guides.github.com/introduction/git-handbook/;
- a beginner's tutorial: http://ryanflorence.com/git-for-beginners/;
- a simple git guide: https://rogerdudler.github.io/git-guide/;
- an interactive tutorial: https://learngitbranching.js.org/. The relevant parts for you are in the "Main" section, introduction sequence, parts 1-3, and in the "Remote" section, the full "Push & Pull" sequence. Feel free to do the other parts if you want :). You can also go to https://learngitbranching.js.org/?NODEMO if you want to just play around;
- the Pro Git book: https://git-scm.com/om/book/en/v2 , you'll find pretty much everything about git there, but might be tedious :);
- a list of some graphical interfaces for git: https://git-scm.com/downloads/guis;
- for when things really go wrong:https://ohshitgit.com/ (be careful using this!).
A couple of videos recommended by class10 student Lilla Kőrösi:
Video about submitting your homework using Pull Requests:
And finally, atlassian (the company that makes BitBucket) has really nice tutorials as well. Check them out here and explore the stuff in the side bar :).