CPanel is the name of the interface that allows you to manage your web site from the web host's backend. It is through CPanel that you can see how much disk space your site is using, manage your e-mail accounts, set up FTP accounts, access the files being hosted if you like. And it is through CPanel that you can easily install Joomla!, Wordpress and a bunch of other stuff.
CPanel is accessed through your web browser. In your "welcome" letter you will have received the URL for your particular installation of it and you will have received the username and password with which to login.
Once logged in you will see general information about the web hosting business side of your site on the left side of the screen. Things like disk space used, number of subdomains allocated, bandwith used can be seen there. The majority of the screen is a bunch of icons representing different things that you can manage. The ones you will most likely most use are the "Mail" (to set up e-mail accounts and e-mail forwarding), "Parked Domains" (if you wish to have several domains pointing to the same "main" site), "Subdomain" (if you wish to allocate subdomains), "FTP Manager" (to see FTP account information), "File Manager" (to see, manage, edit the files of your site) and "Fantastico" (to install or uninstall things).
Most of the functionality is actually rather simple but sometimes the language used to describe it is unfamiliar and the feedback around any actions you might take is not as friendly as I at least would have it. But... you should feel free to explore and try things "anyway". This is your site. The more you learn about it, the more you will feel in command of it. Directing your intention is always a good thing :o)
There is a manual for CPanel available at http://www.cpanel.net/docs/cpanel/.