cPanel is a web hosting account management tool available on all web hosting plans.
You only get one username per cPanel. If you create addon, parked and subdomains, those domains can be accessed by logging in to your main cPanel; you will not have a separate cPanel or separate login credentials. If you need seperate usernames, then add another web hosting service to your account with us.
To access cPanel, go to your Dashboard. Under Services, you should see a service that reads "_____ Web Hosting". Click anywhere on that line (except for manage), and you should see the details pane open up right below it. Click the "Log In" button, and you will be automatically logged into cPanel.