(Once your reseller account is activated, you can use the same credentials as your primary cPanel login. )
WHM is accessible through two methods. The first is via the 2086 port and the second is by your website URL followed by /whm.
For example: If your website is yourdomain.com, You can access WHM in the two following manners - https://yourdomain.com:2086 or https://yourdomain.com/whm
username: given to you through your email, same as cPanel credentials
password given to you through your email, same as cPanel credentials
VPS and Dedicated customers: Your username is root