If you want to purchase a .com domain, you do that through a domain registrar. You can Google for some, but many use GoDaddy.
If you have already bought a domain, you can either use the Admin Panel to change your main domain to that domain or use the cPanel to park it or add it as an add on. Before you do that, you tell your domain registrar to set the nameservers x10's (or set the A record to the IP of your server). There is no charge by x10 for hosting a .com domain.