If you changed your SSH key or just created it but still not cloning:
$ git clone ssh://firstname.lastname@example.org/youruser/userproject.git Cloning into 'yourproject'... Agent admitted failure to sign using the key. Permission denied (publickey). fatal: Could not read from remote repository. Please make sure you have the correct access rights and the repository exists.
Then you could check if your computer key is accepted by github:
$ ssh -T email@example.com --version Agent admitted failure to sign using the key. Permission denied (publickey).
Then you just need to force ssh to load your new KEY this way:
$ ssh-add ~/.ssh/id_dsa
DONE! Now you can clone and push your project from and to github.