Logging in to the WHM (Web Host Manager) control panel is very simple. In your web browser's URL bar, type one of the following URL's:
https://youripaddress:2087 (SSL secure)
Assuming your are logging in as the root user, your username would be 'root'. Your password would be the root password sent to you in your welcome email.
You will need to substitute the main IP of your VPS for 'youripaddress' above. If you choose to log in using SSL encryption and have not yet configured an SSL certificate for your VPS signed by a trusted authority, you may need to opt to continue past any security warnings that may pop up, depening on how your web browser is configured.