I haven't set up keys or did something similar, I just typed command that I mentioned above.
And that’s the problem with blindly following suggestions in answers to completely different questions :)
You basically told Git now to connect and authenticate via SSH but didn’t give it a single information on how that actually works. SSH uses a a private/public key combination which requires you to generate those first on your machine and upload the public one to GitHub then.
Please see this help document by GitHub on how to set up your SSH keys correctly and how to add the public key to your account.