GitHub Desktop is a graphical user interface which mimics the functionality of the GitHub website, but runs on your computer.
GitHub Desktop can be downloaded from GitHub's website:
The GitHub website has documentation relating to GitHub Desktop:
Getting Started with GitHub Desktop describes how to set up the program, connect it to your GitHub user account, and other basic set-up tasks.
Contributing to projects with GitHub Desktop describes how to use the program to contribute to your own or other projects hosted on GitHub. This documentation include descriptions of how to add and clone repositories, how to make changes to a branch, and how to make sure the repository on the GitHub website stays synchronized with the version on your computer.