Please remember that a shared plan cannot add a second domain for hosting. Even domain aliases are not allowed.
Any other plans allow an unlimited number of additional domains and domain aliases.
Shared Web Hosting
You will add extra domains through your cPanel feature called "Addon Domains". Please read about Addon Domains here.
Reseller and Dedicated Hosting
You can add extra domains through your WHM feature called "Create a New Account". Read about how to "Create a New Account" here. You can also allow each cPanel user to add extra domains via the "Addon Domains" feature, and the limitation of this feature is decided by you.
Don't forget! You must register a new domain name before you can use it with your hosting. All domains added to your account use the same name servers we provided in your welcome email.