By default, Debian has an ssh client, but not an ssh server. So you can ssh
out of a Debian box as soon as it's installed, but you can't ssh
into one. As lucazorzi says, you need to install a server. You can do it as easily as this:
Code:
aptitude install ssh
You need to log into a terminal as root to do that, by the way. Enter
su and then your root password when it asks for it. Then do the aptitude command.