To find your cPanel, simply go to http://domainname/cpanel, replacing the "domainname" part with your actual domain name, with or without www.
On doing this, you will be redirected to SSL.
You will then be presented with a login page where you can login with your username and password, and begin working with your cPanel.
When you first log in you will be presented with the option of viewing some tutorials that give you detailed instructions.