|It depends on who your host is. |
When you signed up, they should have sent you an email and/or revealed a page that contains the information required of you to log in and what url is to be used. As an example, if your host is xyz.com then the cpanel *may* be cpanel.xyz.com
If you know your account id and password, then if you have ftp access, you can use an ftp client such as WS_FTP to log into your account.
If that still doesn't help, then I'm really not sure how to offer assistance. Good luck.