You will need to edit
/etc/ssh/sshd_config as follows:
# Both of these are probably already there, but commented
# The next line makes sure that sshd will look in
# $HOME/.ssh/authorized_keys for public keys
Additionally, if you want to disable password authentication alltogether (which is usually a good idea, if you use keypairs), add the following:
# Again, this rule is already there, but usually defaults to 'yes'
After that, restart ssh by issueing
/etc/init.d/sshd restart and you should be fine!
The above assumes you have already properly created the
.ssh dir with the proper permissions.
This means you set
chmod 0700 to