Checking for existing SSH keys:

https://help.github.com/articles/checking-for-existing-ssh-keys/

Generating a new SSH key and adding it to the ssh-agent:

https://help.github.com/articles/generating-a-new-ssh-key-and-adding-it-to-the-ssh-agent/

Adding a new SSH key to your GitHub account:

https://help.github.com/articles/adding-a-new-ssh-key-to-your-github-account/