This is really simple.
We have chosen OpenSSH as the best software for the job.  Begin by opening a Terminal Window on the server.  Enter the following command to install OpenSHH:
sudo apt-get install openssh-server

Done!

To test that it’s working, you can try to ssh into the server:

ssh localhost

You will be prompted for a password.

Then exit the SSH session by typing:

exit

See the screenshot below:

Now, if you want to access the server remotely, you will have to set up port forwarding on your router to get outside connectivity to the SSH server.  We won't discuss that here but you must have a fixed IP address on the Ubuntu server and then configure your router to forward incomming SSH requests to the fixed IP.