You need to download GitHub first and foremost, from here.
Create an account on GitHub.
Once done, navigate to here to create a new repository.
Call it whatever you want.
Then go into the repository, the URL should be similar to: https://github.com/user-name/repository-name there's a green button called "Clone or Download", click it and copy the URL.
I'm a Windows user so I will be only showing the how to for Windows.
Press the Windows key and type "cmd", selecting the GitHub command line interface.
Choose where you want to store the Git project you're going to connect to.
Using "cd" and all that jazz to navigate, "dir" to list etc.
In the chosen directory paste: git clone https://github.com/user-name/repository-name.git
This pulls down the contents from that location to your specified location.
Now add a file into the location on your machine, text file or whatever.
Paste and run: git status
This will specify the differences between the files on your local machine and what GitHub has.
Paste, edit, and run: git add your-file-name-and-extension
Paste and run: git status
This will tell you what's been added.
Paste and run: git commit -m "added test document through cmd"
Where "-m" enables you to write a message to the file(s) you're proposing to add.
Finally, paste and run: git push
This may ask you for user name and password, once authenticated the changes will be uploaded to GitHub to be seen through the browser.