To log into GoDaddy you either have to use your Customer ID or Username. The best way to find your Customer ID is to check your emails for any email from GoDaddy, at the top of the email you will see something similar to this:
Then simply enter that number into the UserName field.
If you cannot find an email they you are able to fill out a form and request your username. Click on the link below and follow the steps, it will ask for a domain name under the account and also your email.