It is important to choose a hostname that you will not be using for any account. You should choose something that will be socially acceptable, as it will appear in all mail headers. Your hostname must be a FQDN.
The hostname of this server should be a Fully Qualified Domain Name that will not be used elsewhere on the server. You should never set your hostname to an name that is already used (www, ftp, mail, etc) or to the name of an account on the server (www.domain.com). This hostname needs to have an A Record and be in DNS to be valid. There is a setting in your WHM (toward the top) where you can imput your hostname to change in on your server.