In order to create a mailbox you will firstly need to be logged into the hosting control panel, from there click on the option
Email > Email Accounts:
From the subsequent page you will be given the option to pick your mailbox name (just the part before @domain.com is needed) and create, a random password will be generated for you:
If you wish to update to your own password please scroll down to the [b]Modify email accounts[b] section and you can update the password from there: