CONFIGURING THE SSH SERVER
First step, we need to create a backup of the configuration file, so we can reset quickly the basic configuration because during editing important informations may be lost.
$ sudo cp /etc/ssh/sshd_config /etc/ssh/sshd_config.backup
Copy also the file containing the security key automatically generate by OpenSSH. You can arbitrarily choose this folder.
$ sudo cp /etc/ssh/ssh_host*_key* /directory/backup
Now open sshd_config with nano (text editor), and we can easly modify it, because it's all commented.