New MacBook had user accounts already; I deleted admin and can't get back in So when I get this new machine...there is already a user on it called "mac"...for my convenience they said. There is also another user already there named "admin"

well..i'm new to apple but when it's my pc, I am the admin. so i erased the "admin" they made (and for which they gave me no password for this account". 

so i deleted it. now no one can log in, i can't use apple id, I've changed the passwords several time successfully only to attempt to make an edit or log in and get the message that an admin has to give his username and password....but i made myself the admin!!!!! why can't i log in with my username and the correct password i regenerated?

it's been hours of trying to make new admins, resetting passwords, everything... mount -uw, rv/ etc. it is totally messed up.

and this company did this account making for my benefit...? i can't get system assistant to do anything or even activate again. I've reinstalled OS 10.10...nothing. does anyone? know if i have to start at square one, reinstall yosemite or is there a way to fix this? much appreciated.
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.