As a reseller you are able to add a cleints web site files into a cpanel account that you create in your reseller account. To learn how to create accounts and cpanel accounts for your clients please see our Reseller Quick Start FAQ section at:
http://www.greengeeks.com/kb/idx.php/30/0/Quick-Start-Guide.html

The temporary login instructions for you to add your cleints' site files prior to changing the dns can be found in your reseller login instruction email.

Find FTP software by clicking the following link:
http://www.greengeeks.com/kb/idx.php/19/051/article/Where-can-I-find-FTP-Software.html

And in addition you can use the file manager in the control panel to upload the files. There are also video instructions within cPanel which will walk you through how to upload your web site through the file manager. Look for the Video's Icon in the first section at the top when you login to your cpanel account.