When your server is first activated it won't have any websites set up, you need to create the account yourself. This is done through the Web Host Manager:
Under Account Functions in the sidebar click "Create a New Account".
Fill in the account details. WHM will automatically suggest an appropriate username based on the domain name, but you can choose your own if you wish.
If you have setup packages choose which package to assign that account. Otherwise click Select Options Manually to enter the account's resources.
Make a note of the nameservers, you will need these later.
Click the Create button.
You will then receive an email confirming the account details.
If the account uses a domain name, you will then need to change the nameservers to match those you noted down earlier.
Congratulations, you've just created your first account. You can now login to cPanel and begin managing your website.