To access your Control Panel (cPanel) -
Make sure that the url is correct. It should read something like this -
Your IP address to access your server is also in your welcome email.
Make sure that you are using the right Username and Password when the Password box appears on the screen. Your Username and Password is CASE SENSITIVE, so make sure to be exact here.
After your domain name has propagated, you can access your Control Panel via http://yourdomain.com/cpanel
Before your domain name propagates, you must access your Control Panel via http://IPADDRESS/cpanel