I'm not sure if I get your question right, but in
/var/log/auth.log are (beside other things) log entries coming from
sshd. I think you want to create a logfile with only those entries from
sshd to see who and when is accessing your server via ssh, right?
sshd uses syslog to do the logging, in your case
rsyslog. This logging is configured in
/etc/sshd/sshd_config. The relevant settings are
LogLevel. Look at the manpage to understand what they do.
To get a single logfile
/var/log/sshd.log which only contains messages coming from
sshd you have to modify the rsyslog configuration. Create a file
/etc/rsyslog.d/sshd.conf and put the following line in it:
if $programname == 'sshd' then /var/log/sshd.log
sudo service rsyslog restart