Contact the seller and have him send you the discs that came with your computer. Otherwise, you can call Apple and order replacements:
Apple Store Customer Service at 1-800-676-2775 or visit online Help for more information.
To contact product and tech support visit online support site.
If the computer has Lion or Mountain Lion installed, then try this:
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
Do the following to reset the admin password:
- Boot into single user mode by holding down Cmd-S as the computer boots.
- Type fsck -fy.
- Type mount -uw /.
- Type rm /var/db/.AppleSetupDone.
Reboot and you will be prompted to create a new user account. However I would advise following Kappy's advice to contact the seller.