Your host should be able to help you, but you should have been provided with WHM (Web Host Manager) details.
Your login for WHM should the same as your cPanel login, and typically can be accessed from yourdomain.com/whm
Once you've logged in you'll want to create some packages, or just create an account and it'll make the package for you.
A package is simply a set of features for an account. So for you could have a Starter package which only allows one domain and a little space, and then a Business package which allows multiple domains, greater resources and so forth.
Each account you create in WHM will then have it's own cPanel login details, which your client - end user will use to manage their account.
Master reseller simply means you can sell reseller accounts where your client can also provide reseller accounts. This is can get pretty messy though, because the chain can be limitless!
I'd probably recommend you stick to creating end user accounts in WHM for now, to sell normal hosting packages, and then move on to providing reseller accounts once you've learnt the ropes.
Here's the WHM documentation:
http://docs.cpanel.net/twiki/bin/view/11_30/WHMDocs/WebHome
If your host is any good they should help you get set up. WHM can look complex at first if you're moving from only having used cPanel before, but it's nice software. Just have a look around.
Good luck
