You need to follow this instructions https://cloud.google.com/compute/docs/instances/connecting-to-instance#generatesshkeypair
If get "Permission denied (publickey)." with the follow command
ssh -i ~/.ssh/my-ssh-key [USERNAME]@[IP_ADDRESS]
you need to modify the /etc/ssh/sshd_config file and add the line
AllowUsers [USERNAME]
Then restart the ssh service with
service ssh restart
if you get the message "Could not load host key: /etc/ssh/ssh_host_ed25519_key" execute:
ssh-keygen -A
and finally restart the ssh service again.
service ssh restart