Create a new repository, or reuse an existing one.

Generate a new SSH key:
```
ssh-keygen -t rsa -C "your_email@example.com"
```

Copy the contents of the file `~/.ssh/id_rsa.pub` to your SSH keys in your GitHub account settings (https://github.com/settings/keys).

Test SSH key:
```
$ ssh -T git@github.com
Hi developius! You've successfully authenticated, but GitHub does not provide shell access.
```

Change directory into the local clone of your repository (if you're not already there) and run:

```
git remote set-url origin git@github.com:username/your-repository.git
```

Now try editing a file (try the README) and then do:

```
$ git commit -am "Update README.md"
$ git push
```

You should not be asked for a username or password. If it works, your SSH key is correctly configured.