Welcome aboard!!
I'm no pro and relatively new to vps myself.
But one of the first things if you haven't done it yet is updating your dns/names servers at your register.
To create an account for your main domain/yourself you would want to login in to whm and set yourself up with an account. Even though your vps might be host.yourname.com you still want to add your domain with whm yourname.com that will automatically put your account/files in the /home directory in vps virtuozzo.
When you first login to whm it will ask you to setup a bunch of things or confirm them anyway. Most of which are generally default.
After that and you create an account for yourself with whm, then login to your new cpanel account,
www.yoursite.com/cpanel you can then upload your files just like you would at your shared provider. Any databases same way download them from your old host and upload them with the your new cpanel account you created for yourself.
It takes a little time and playing to figure out, I think you'll find you learn something new everyday.
As for the 20% space used depends on your package you have and the control panel you are using, between them and the operating system itself linux centos most likely, it can consume 1-3 gb's of your space. The new cpanel 11 and the updates have put a fairly big dent in my available space.
gameutopia