You'll need to get a paid Google Apps for Work account at google.com/work/partners to apply.
Register at least 100 licenses across one or more customers at google.com/work.
|Credit check is the final step to authorization. You'll need certain information depending on your region.*|
Upon approval, accept your reseller agreement, and start provisioning customers.
Note: The minimum number of licenses will change to 100 on August 1, 2015 (FAQ).