how to create a cron job that runs on the first day of month [duplicate]

This will run the command foo at 12:00AM on the first of every month

0 0 1 * * /usr/bin/foo

This article describes the various fields, look to the bottom of the page: http://en.wikipedia.org/wiki/Cron

To add this to your cron file, just use the command

crontab -e