I don't have administrator account on my mac
You can create a new administrator account by restarting the Setup Assistant:


Boot into Single User Mode: Start/restart your Mac. As soon as you hear the startup tone, press and hold ⌘ + S until you see a black screen with white lettering. (If you end up back on the login screen after a flash of the black screen with white lettering, enter your password and it will return to the black screen.)
Check and repair the drive by typing /sbin/fsck -fy then ↩ enter - as directed by the on-screen text.
Mount the drive as read-write by typing /sbin/mount -uw / then ↩ enter.
Remove the Apple Setup Done file by typing 
rm /var/db/.AppleSetupDone then ↩ enter.
Reboot by typing reboot then ↩ enter.
Complete the setup process, creating a new admin account.


Note that this may result in the new account being logging in automatically when the system is restarted, replacing whatever account may have been set to automatically log in previously. If you need to retain access to the original account without knowing its password, this may mess that up.