Go into cPanel and set up your domain as a parked domain (if you want it to display the same pages as your *.x10hosting.com one) or an addon domain (choose this if you want it to be different to your *.x10hosting.com one). Then go to your registrar's website and change the nameservers to ns1.x10hosting.com and ns2.x10hosting.com. Unfortunately there's not much we can do to help you here, as all registrars use different systems. If you need support changing the nameservers, contact them and ask them for help.
Note that it can take up to 72 hours for your domain to work for everybody, it's called DNS propagation.
-Luke.