Hello Raghava,

Ideally, the user name of system administrator will be "admin" (unless you changed this on your system). For password, you can change it form the user record or reset it from developers portal by following the below instructions.

Under the My Instance page, use the "Action" button and select the "Reset admin password" action. After confirming that you wish to proceed, the developer site resets the admin password and provides you with a temporary password in green text that you should copy. Navigate to your instance by clicking the "URL" text in the green box or typing the URL in a new tab or window. You are prompted on your personal developer instance to set a new password. Use the temporary password for the current password and provide a new password twice.