New MacBook had user accounts already; I deleted admin and can't get back in

Solution 1:

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.

Solution 2:

Two things:

1. Depending on where you were sold the Mac - you may have a legitimate claim for money back or a total refund. Bad sellers are no help generally, but perhaps they aren't bad and you could ask them for help - if they are reputable, they may bend over backwards to make you happy.
2. Apple supports their hardware no matter how you got it. I'd start with AppleCare and above all else - make a full back up of the Mac before you erase it.

You can use that backup to see what apps were bundled (like Pages/Numbers/Keynote) and also iPhoto/iMovie/GarageBand that you might have to pay for if you don't have backups of them.

I personally wouldn't trust an OS I didn't install, so long term, you will want to erase it and start with just the things you can trust from the backup.

To get immediate admin control of the Mac - write down all the folders in /Users and then power off the Mac. Hold command S and follow the article below up to the two commands: https://support.apple.com/kb/HT2600

fsck -fy /  
mount -uw /  

Then do these two commands:

rm /var/db/.AppleSetupDone
halt

Power up your Mac and then make a new account with a name different than the ones you wrote down previously. This account can be used to reset the passwords of the "admin" account or delete it. Deleting all other accounts won't delete the apps - just the settings. You can then set up a new admin account if you want.

Solution 3:

To do it, first type ⌘+R at startup. Then move your mouse cursor to the top and look for Terminal. Open Terminal and type this: resetpassword
Then on the account you need, you can perform the function.