What is the correct URL to login to webmail?

You can either login via your control panel, or by visiting http://yourdomain.com/webmail/ 

Change yourdomain.com to what your website address is, so in the exable above we are assuming you get to your website by going to http://yourdomain.com

The username would be the FULL e-mail address for that account that you would use if you were sending email to it and the password would be the password that person uses to access their email.