GitHub: Permission denied (publickey). fatal: Could not read from remote repository

I´m having trouble with GitHub. I´ve playing around with a remote repository of Git.

When I now try to make any changes to the remote directory, i.e.

git remote show origin

or

git push -u origin master

I get this error

Permission denied (publickey). fatal: Could not read from remote repository.

Please make sure you have the correct access rights and the repository exists. Ulrichs-MacBook-Pro:coredatatest ulrichheinelt$ git push -u origin master Permission denied (publickey). fatal: Could not read from remote repository.

I would be happy, when I could start again with a new (empty) remote directory. Or is there a way, to fix this error?

This are my first steps with GitHub, started yesterday...

Edit 1

my settings at https://github.com/UlliH/CoreDataTest/settings

...

Edit 2

too early happy :-(

After setting the SSH and GPG keys, the errors are still the same. :-/

Edit 3

I think that's right so, but still the same...

• On your GitHub profile there is an Edit Profile button.
  It is located on top-right corner of the webpage.
• Press it and you will see left Personal Settings menu.
• Inside that menu find SSH and GPG keys option and press it.
• You will see an option New SSH key to add new key.