CPanel is your hosting account's control panel, which allows you to control many aspects of your hosting & it's features, all via your web browser.
To login to your CPanel, go to http://yourdomain.com/cpanel
If you are behind a firewall and can't access the above link, try http://cpanel.yourdomain.com
For brand new accounts which are still waiting your domain name to propogate, you can access using your hosting IP address (see your signup email):
Your CPanel login details were sent to you in your hosting signup email.
If you require the password to be reset, contact firstname.lastname@example.org
** Just be aware if your CPanel password is changed, it will also change your main FTP login password.