On a Debian system, you probably already have an SSH client (openssh-client) installed but on the box you want to log into remotely, you need to have an SSH
server installed and running.
On these pages, let's call the remote/server machine 'remotebox' and the client machine will be 'localbox'.
So, on remotebox, use your favorite package manager and install openssh-server.
When that's done, type su -c /etc/init.d/ssh start to make sure it's running.
The first time you start an SSH server, it builds some keys. It only does this once, so subsequent starts will be quicker.
Now you can move back to localbox and make sure you have a client installed. (Typing “which ssh” is a quick way)
If you need the client, use your favorite package manager and install openssh-client.
Now, at a shell prompt type ssh user@ipaddy (user being a user account name on the remotebox and ipaddy being remotebox's ip address)
On first time logins, you will see a message about unknown hosts. Answering yes will add the server's public key to localbox's ~/.ssh/known_hosts.
If all went well, you are now logged into a shell on remotebox and are free to issue commands as if you were sitting at remotebox.