If it's a new machine, you have to setup an account for yourself and a new password. In order to do that, just press enter for the time being for both boxes (name and password). Then open System Preferences, click on Users & Groups. Click on the small lock at the bottom, press enter for it to open. Now you can assign a name and password for yourself and give it Admin privileges.
Write down the password somewhere in case you forget it. From now on when you log on to your new Mac, enter your name and password.
By the way, the first time you turned on your new Mac, it should have asked for the above (new name and password). Did you miss that?