Presuming from the tag you're talking about the Best Users Per Month plugin. If you have direct access to the server you can run the command
crontab -e
on the server command line. Then you'd need to put a line like
0 0 1 * * php /path/to/crontab.php
The readme for that plugin has some info about using cpanel to set up the cronjob if you have that.
Otherwise if you host gives you no option to make a cronjob, you'll have to find another way. You could load the script manually on the first of each month. Or you can use scheduled tasks on your own computer.
