I just reset my iMac and i do not remember creating password for system preference. Ho do i unlock my computer

The password is not made to easily get around !

If you feel confident in the command line you can do this:

Hold down CMD+S while booting (boots into single user mode),

mount the drive by typing or copy/paste:

mount -uw /

then hit return, and then type or copy/paste:

launchctl load /System/Library/LaunchDaemons/com.apple.DirectoryServices.plist

(this is one line command) and hit return, and then type (copy/paste):

ls /Users

and hit return, note the short name (username) of your account, then type (copy/paste)

dscl . -delete /Users/username AuthenticationAuthority

replace username with the noted username above, then hit return, and then type (copy/paste) this:

passwd username

again with your noted name instead of username , and hit return;

Now you get a "new password:" prompt, type the new password, then hit return,

on "Retype new password:" prompt retype it and hit return

THEN type

reboot

and hit return.