Did you already go through the Setup Assistant? This is what you go through when first turning on the new computer. If so then you were asked to provide a username and password when creating the initial user account. Do you remember what you set up at that time? If you don't even know what the username was then you might try this:
Forgot Your Account Password
For Snow Leopard and earlier
For Lion/Mountain Lion
Boot to the Recovery HD:
Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
When the menubar appears select Terminal from the Utilities menu.
Enter resetpassword at the prompt and press RETURN. Follow
instructions in the dialog window that will appear.
Or see Reset a Mac OS X 10.7 Lion Password and
As a last resort boot into Single-user Mode. When the prompt appears enter the following commands. Press RETURN after each:
mount -uw /
shutdown -r now
When the computer restarts it should default to the Setup Assistant. You can then create a new user account with a new username and password.