Ensure the OS is up to date.
sudo apt-get update
Install OpenSSH server on the OS.
sudo apt-get install openssh-server
It's also a good idea to make a copy of the default SSH configuration, so that if you ever mess up your SSH settings you can revert to the original.
sudo cp /etc/ssh/sshd_config /etc/ssh/sshd_config.factory-defaults
That's all you need in order to get SSH working. You should now be able to use a program like Putty in order to connect to the remote OS via port 22 (assuming you have left all defaults).
If you have changed anything in the config file remember to restart the SSH service.
sudo restart ssh