It has to be noted that GitHub has grown in both size and popularity and is a great place for working on open source projects. Suppose though you want to do more than just raise issues, well, see below for some help.
If you want some help or power tips then tiimgreen/github-cheat-sheet is a great place to start.
Suppose you find a repository and you would like to contribute. It is unlikely the owner will give you direct contributor access, so try something like this:
It is simple until such a time as you find the repository that you forked has moved on and you want to do another change. If this happens, then you need to do the following on the machine you did the changes:
git fetch <repository>- this pulls the original repository changes down
git checkout master- should just confirm you are already on "master"
git merge <repository>/master- merges the changes to your local git
git push- pushes your local git up to GitHub
This is a short summary of what I did to create a Pull Request with the Desktop App for Windows, it confused me at first but the key is you need to work from your own branch or fork.
You will have noticed that many repositories have a README.md file, this is a "markdown" file and forms the introduction to the repository. If you need help with the syntax see Writing on GitHub - User Documentation which covers everything but Mastering Markdown · GitHub Guides is a great starting point. Do note that GitHub does not use standard markdown but "GitHub Flavored Markdown" or GFM. See also File Formats.