Specifying the private key to use in ssh
A private key is necessary in ssh to authenticate the connection securely.
Generally ssh will look for a file in the ~/.ssh/ dir with a name of id_rsa
(or if not using rsa a file beginning with id_ and the encryption algo name). That file should contain the private key.
You can, however, specify a different primary key at the command line with the -i flag like so:
ssh -i ~/.ssh/my_other_key.pem
Additionally, you may use the IdentityFile option the the ssh_config file to permanently configure the private key for a specific connection.