New MacBook had user accounts already; I deleted admin and can't get back in
I would suggest you to reinstall OS X and doing it not on the top of the existing version.
Start Mac in Recovery mode (hold Cmd-R keys pressed on boot). When in Recovery mode - select Disk Utility, format your disk, exit Disk Utility and select Install or reinstall OS X.

P.S. If you have your files stored in Mac - backup ones before formatting the disk.