Your cPanel is accessed by adding /cpanel/ on to your domain name. So, if your domain name is yourdomain.com, your cPanel is accessed at http://www.yourdomain.com/cpanel/
If you've just set up your account, your domain name might not work just yet as it may not have propagated to your ISP. To access your cpanel in this situation, read you introduction email we send with your details in it for which IP address your server has. Your cPanel is then at something like http://123.456.789.123/cpanel/
Another alternative to /cpanel/ is to use the 'port number'. I'll not explain what 'port numbers' are but it's sometimes useful if you've somehow managed to install a mod_rewrite rule which is conflicting with /cpanel/. This alternative method is to add :2082/ to your domain name or IP address. eg. http://www.yourdomain.com:2082/
Regardless of which method you use, when asked, enter in your user name and password. These are given to you in your introduction email.