It is simple. There are two ways to do it:
1. Type http://www.yourdomainname.com/cpanel in the address bar of your browser window and enter. You will be asked about the login name and password. Fill in the fields with required information and hit enter. You will be taken to your control panel.
2. When you purchase a hosting account or domain, an email is sent with details of your client area and hosting account on the registered email ID with us. You will find your login information there. What you can do is to simply log in to your client area from http://www.centroot.net/ or http://www.centroot.net/whmcs/ all your information is kept there and from there you can perform multiple operations.
In case you have missed this information, you can request it by opening a ticket at http://www.centroot.net/whmcs/