SSH is a great way to remotely administer a server. However, it still has a number of issues when you open it up to the world. The server and client communications are secure but that doesn’t mean the hosts involved are. Opening an SSH service to the world allows for brute force attacks and means that the carbon interface is still the weakest link.
First things first, sshd_config. In Ubuntu this is usually found in /etc/ssh and can be used to configure a great number of features.