First run ...
ssh-keygen -t rsa -b 4096 -C 'email@domain.com'
... from terminal, then run ...
eval "$(ssh-agent -s)"
... then run ...
ssh-add ~/.ssh/id_rsa
then copy the content of ~/.ssh/id_rsa.pub
and add it to github under Settings >> SSH and GPG keys >> New SSH key,
then run ...
ssh -T git@github.com
and confirm!