The control panel is already installed, you do not install it.
Your Control Panel is not accessible via http://yourdomain.com/cpanel until it resolves across the Internet. this usually takes 24 hours or so.
However, you can access it via it's IP address BEFORE IT RESOLVES, which is in your welcome letter. The IP/cpanel method would look similar to:
https://66.246.xxx.xxx/cpanel. (replace the x's with your actual IP)
But check your welcome letter just to make sure.