When cloning a GitHub repository you can choose either https or ssh. I will explain both methods:

Alt 1: Clone a GitHub repository with https

git clone https://github.com/username/reponame.git

Alt 2: Clone a GitHub repository using SSH

In order to use SSH you first have to exchange keys. If you haven't already, take a look at this post:

git clone git@github.com:username/reponame.git

You may get this warning:

The authenticity of host 'github.com (140.82.121.3)' can't be established.
RSA key fingerprint is ....
This key is not known by any other names
Are you sure you want to continue connecting (yes/no/[fingerprint])?

Write yes and press enter.

List files to see if the repo was successfully added; "ls" on Linux, "dir" on Windows.