crontab @reboot /home/admin/script.sh
To setup a crontab entry you run "crontab -e" which puts you into an edit (vi) session. You then input your detailed entry:
and save then exit.
That adds the entry to the crontab file in /var/spool/cron for the user you ran the command as. (e.g. /var/spool/cron/root if you did it as the root user.)
The @reboot per "man 5 crontab" replaces the standard 5 date/time specifications so you don't add the * * * * * in addition to the @reboot.