See this page for more details on how to install OpenSSH Server:
https://help.ubuntu.com/community/SSH/OpenSSH/Configuring

Run the following command to install OpenSSH Server:
$ sudo apt install openssh-server

Use SSH to connect to the server:
$ ssh mtitek@localhost
You may get the following error:
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ @ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @ @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY! Someone could be eavesdropping on you right now (man-in-the-middle attack)! It is also possible that a host key has just been changed. The fingerprint for the ECDSA key sent by the remote host is SHA256:Dmz1/rPX9LcCQjFzNSvVsvB/rBBlYaVcp9rqGv9f1gU. Please contact your system administrator. Add correct host key in /home/mtitek/.ssh/known_hosts to get rid of this message. Offending ECDSA key in /home/mtitek/.ssh/known_hosts:3 ECDSA host key for localhost has changed and you have requested strict checking. Host key verification failed.
To fix this error, open the file '/home/mtitek/.ssh/known_hosts' and delete the line corresponding to the host you are trying to connect to (localhost).