Changing The Administrator name and password I just bought a used iMac. Should arrive Friday morning. I just thought of something... This is a used computer, so how am I going to delete certain files if I don't know the Administrator name or password?
You can boot into Recovery (hold Command + R during startup) to change the password. Once booted from the recovery partition select Terminal from the Utilities menu and then enter resetpassword

Although, honestly, you might just want to erase the disk and install a new copy of OS X.