First of all:
Create a simple user with ssh login:
- $ useradd -m -s /bin/bash username
- $ passwd username
Open a second shell and log the user in. Try command
$ su
to check, if your user can login as root as well.
After successful test ...
Open /etc/ssh/sshd_config
Set "PermitRootLogin no"
Reload configuration with
/etc/init.d/ssh reload
Open a third shell and try to login as root -> should not be possible anymore.